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Properties and Characteristics of Graphite

Properties and Characteristics of Graphite



For the semiconductor industry May 2013 INTRODUCTION

Introduction Table of Contents As Entegris has continued to grow its market share of Introduction ...... 1 manufactured graphite material and to expand their usage into increasingly complex areas, the need to Structure ...... 2 be more technically versed in both Entegris’ POCO® graphite and other manufactured graphite properties Apparent ...... 6 and how they are tested has also grown. It is with this Porosity ...... 8 thought in mind that this primer on properties and characteristics of graphite was developed. Hardness ...... 11 POCO, now owned by Entegris, has been a supplier Compressive Strength ...... 12 to the semiconductor industry for more than 35 years. Products include APCVD wafer carriers, E-Beam cruci- Flexural Strength ...... 14 bles, heaters (small and large), ion implanter parts, Tensile Strength ...... 16 LTO injector tubes, MOCVD susceptors, PECVD wafer trays and disk boats, plasma etch , Modulus of Elasticity ...... 18 replacement parts, sealing plates, bonding fixtures and sputtering targets. Entegris has a wide range of Electrical Resistivity ...... 19 materials, as well as post-processing and machining Thermal Expansion ...... 21 capabilities to meet the demanding requirements of semiconductor processing. ...... 22 With an accumulation of experience and data in ...... 24 testing POCO graphites as well as other manufactured graphites, it is now possible to describe the properties Specific Heat ...... 26 and testing techniques and their interrelationship. Emissivity ...... 28 The very discussion of these issues will bring forth many of the reasons why POCO graphites with their Ash ...... 30 unique properties are superior to other manufactured graphites. Oxidation ...... 32 The purpose of this primer is to introduce the reader Appendix A ...... 34 to graphite properties and to describe testing tech- Appendix B ...... 35 niques, which enable true comparisons between a multitude of manufactured graphites. POCO graphites Appendix C Bibliography ...... 36 come in many grades, each designed for a specific range of applications. In the semiconductor industry, For More Information ...... 38 these are represented by grades such as SFG, CZR, Terms and Conditions of Sale ...... 38 TRA, DFP, HPD, PLS, SCF and ZEE. These are the materials upon which POCO facility has built its Product Warranties ...... 38 reputation as a manufacturer of the best graphites in the world.


Structure TABLE 1-1. PROPERTIES OF THE ELEMENT Name: Carbon Definition: Carbon, the Element Symbol: C Carbon is the sixth element on the periodic table and Atomic number: 6 can be found in abundance in the sun, stars, comets Atomic mass: 12.0107 amu and atmospheres of most planets. Melting point: 3500.0°C Carbon is a Group 14 element (on older periodic 3773.15 K tables, Group IVA) along with , germanium, 6332.0°F tin and . Carbon is distributed very widely in Boiling point: 4827.0°C nature (Figure 1-1). 5100.15 K 8720.6°F Number of protons/: 6 Atomic number 6 12.011 Atomic weight Number of : 6, 7, 8 Classification: Non-metal C structure: Hexagonal Carbon Cubic 4 Common oxidation state Density @ 293 K: Graphite – 2.26 g/cm3 – 3.53 g/cm3 Figure 1-1. Carbon as on the periodic table Color: , gray

In 1961, the International Union of Pure and Applied The history of manufactured graphite began at the Chemistry (IUPAC) adopted the 12C as the end of the 19th century with a surge in carbon manu- basis for atomic weights. Carbon-14, 14C, an isotope facturing technologies. The use of the electrical with a half- of 5730 years, is used to date such resistance furnace to manufacture synthetic graphite materials as wood, archeological specimens, etc. led to the development of manufactured forms of car- Carbon-13, 13C, is particularly useful for isotopic label- bon in the early part of the 20th century and more ing studies since it is not radioactive, but has a spin recently, to a wide variety of high-performance materi- I = 1⁄2 nucleus and therefore a good NMR nucleus. als such as and nanotubes (Figure 1-2). Carbon has four electrons in its valence shell (outer Forms of Carbon shell). The configuration in carbon is 1s2 2s2 2p2. Since this energy shell can hold eight electrons, Carbon is found free in nature in three allotropic each carbon can share electrons with up to four forms: , graphite and diamond. different . This electronic configuration gives More recently, a fourth form of carbon, buckminster- carbon its unique set of properties (Table 1-1). Carbon , C60, has been discovered. This new form can combine with other elements as well as with itself. of carbon is the subject of great interest in research This allows carbon to form many different compounds laboratories today. Within the past few years, this of varying size and shape. research has centered on and its derivatives, which have the potential to bring about a fundamental Carbon is present as in the atmosphere change in the semiconductor/electronic industry. and dissolved in all natural waters. It is a component of rocks as carbonates of calcium (limestone), magne- Carbon alone forms the familiar substances graphite sium and . , petroleum and natural gas are and diamond. Both are made only of carbon atoms. chiefly hydrocarbons. Carbon is unique among the ele- Graphite is very soft and slippery, while diamond is ments in the vast number of varieties of compounds it one of the hardest substances known to man. Carbon, can form. Organic chemistry is the study of carbon and as microscopic , is found in some . its compounds. Natural diamonds are found in ancient volcanic “pipes” such as found in South Africa. If both graphite and diamond are made only of carbon atoms, what gives them different properties? The answer lies in the way the carbon atoms form bonds with each other.


Pre-1880 Lampblack (writing) (, medicine, deodorants) Natural graphite (writing material)

1880–1940 Activated

Carbon 109° 1.54 Å Coal coking (coal-tar pitch) Delayed coking Synthetic graphite and diamond

1940-2013 3.58 Å

Carbon fibers (PAN) Figure 1-3. The crystal structure of diamond Carbon fibers (pitch-based) The forces within and between crystallites deter- Carbon fibers (microporous) mine the extreme difference in properties between Carbon/ composites these two forms. In diamond, the crystal structure is face-centered cubic (Figure 1-3). The interatomic Carbon/carbon composites distance is 1.54 Å with each atom covalently bonded Specialty activated carbons to four other carbons in the form of a tetrahedron. Carbon as a catayst support This interatomic distance is close to that found in aliphatic hydrocarbons, which is in distinction to Carbon whiskers/filaments the smaller 1.42 Å carbon-carbon distance found in Prosthetics graphite and aromatic hydrocarbons (1.39 Å in ben- Intercalation compounds zene). This three-dimensional isotropic structure accounts for the extreme hardness of diamond. Graphite/

Pyrolytic carbon 700 III Mesocarbon microbeads 600 Diamond films Diamond-like films Elastic carbon 500 Diamond

Nanotubes 400 Liquid Nanorods Graphene 300 Pressure (kiloatmospheres)

200 Diamond and Figure 1-2. Growth of carbon materials1 metastable graphite

100 Gr. Graphite and metastable diamond 0 01000 2000 3000 4000 5000 T, K 1 Adapted from Marsh, H. et al., Introduction to Carbon Technologies, (1997), pp. 4, 521. Figure 1-4. The carbon diagram


Thermodynamically, graphite at atmospheric pressure a given plane also provides the electron bond network is the more stable form of carbon. Diamond is trans- responsible for the high mobility (electronic) of graph- formed to graphite above 1500°C (Figure 1-4). ite. This would appear more correct, since van der Waals forces are the result of dipole moments, which would not account for the high mobility. A Consequently, weak forces between layer planes account for (a) the tendency of graphitic materials to along planes, (b) the formation of intersti- c 6.70 Å tial compounds and (c) the lubricating, compressive and many other properties of graphite. B As previously mentioned for the hexagonal graphite d 3.35 Å structure, the stacking order of planes is ABAB, so that the atoms in alternate planes are congruent A (Figure 1-5). Studies have shown that natural graphite a contains 17 to 22 percent of a rhombohedral structure 1.42 Å with a stacking sequence of ABCABC. In “artificial” or 2.46 Å “synthetic” graphite, in the as-formed state, only a few percent at best could be found. However, deformation processes such as grinding substantially increase the Figure 1-5. The crystal structure of graphite percent of rhombohedral structure found in the other- wise hexagonal structure. The structure of graphite consists of a succession of layers parallel to the basal plane of hexagonally Amorphous carbon is also referred to as nongraphitic linked carbon atoms. The ideal graphite structure carbon. When examined by X-ray diffraction, these is shown in Figure 1-5. materials show only diffuse maxima at the normal scattering angles. This has been attributed to a ran- In this stable hexagonal lattice, the interatomic dom translation and rotation of the layers within distance within a layer plane, a, is 1.42 Å and the the layer planes. This disorder has been called tur- interlayer distance, d, between planes is 3.35 Å. bostratic. Some of these nongraphitic carbons will Crystal density is 2.266 g/cm3 as compared with become graphitic, upon heating to 1700–3000°C. 3.53 g/cm3 for diamond. In the graphite structure Some will remain nongraphitic above 3000°C. (sp2 hybridization), only three of the four valence electrons of carbon form regular covalent bonds Thus far, the discussion has centered on the crystal (σ-bonds) with adjacent carbon atoms. The fourth structure of graphites. On a more macroscopic level, or π electron resonates between the valence bond the structure as routinely examined on a light micro- structures. Strong chemical bonding forces exist scope at magnifications of 100, 200 and 500 times within the layer planes, yet the bonding energy reveals the porosity, particle or grain size and the between planes is only about two percent of that general microstructure as it is commonly referred to. within the planes (150–170 kcal/[gram atom] vs. Photomicrographs of POCO DFP-1 graphite compared 1.3–4 kcal/[gram atom]). These weaker bonds to a conventional graphite demonstrate some signifi- between the planes are most often explained to cant differences when viewed at 100× magnification be the result of van der Waals forces. (Figure 1-6) and at 500× magnification (Figure 1-7).

2 It can be seen from these photos that vast differences However, Spain identifies the π orbital, which has a pz do exist in graphite microstructure. These differences configuration, and not van der Waals forces as the cor- are directly related to raw material and processing rect source of bonding between the adjacent layers. In parameters. general, the π bands overlap by ~40 meV to form the three-dimensional graphite network where the layer As seen in the photos, the dark or black regions repre- planes are stacked in the ABAB sequence illustrated sent the porosity while the lighter regions represent in Figure 1-5. Spain concludes in his discussions on the graphite . It is this matrix, composed of electronic structure and transport properties of graph- smaller particles bound together either chemically ite that the overlap of π orbitals on adjacent atoms in or mechanically, which is comprised of the stacked layer upon layer. This is more easily seen in 2 Spain, I.L., Electronic Transport Properties of Graphite, Carbons, scanning electron micrographs (SEM). and Related Materials, Chemistry and Physics of Carbon, 16 (1981), p. 119.


POCO DFP-1 Graphite – mag. 100×

Figure 1-8. The crystal structure of C60 , Fullerene The fourth form of carbon, ,

formula C60, whose framework is reminiscent of the Conventional Graphite – mag. 100× seams in an Association Football (“soccer”) ball Figure 1-6. POCO graphite vs. conventional graphite under light (Figure 1-8), is the subject of considerable interest at microscope at 100× magnification present and was only discovered a few years ago in work involving , a Sheffield University graduate.

Test Methods The structure of graphite has been determined through such methods as X-ray diffraction, transmis- sion electron microscopy, diffraction and convergent beam electron diffraction. These methods are highly sophisticated and generally require very expensive equipment with a highly skilled operator. This is normally beyond the scope of typical industrial laboratories. Since this type of testing or analysis is POCO DFP-1 Graphite – mag. 500× more research-oriented, no standard methods will be presented. However, several books have been published on the structure of graphite and the reader is encouraged to review the bibliography in the appendix.

Entegris’ POCO Graphites vs. Conventional Graphites With regard to crystalline structure, POCO graphite has a typical hexagonal structure. The layer spacings Conventional Graphite – mag. 500× may vary, as they are a function of raw material and Figure 1-7. POCO graphite vs. conventional graphite under light process conditions which vary from manufacturer to microscope at 500× magnification manufacturer. It is reasonable to assume that a certain degree of rhombohedral structure exists also in machined artifacts due to the machining-induced deformation mentioned previously. No testing has been done to confirm this.


POCO graphites are also highly isotropic with respect increase. Conversely to the interlayer spacing, d, to their structure and properties. The isotropy factor is the crystallite size, La, begins a sharp increase about between 0.97 and 1.03 with 1.00 being perfect. A factor 1500°C and continues to about 2000°C where it begins of 1.00 means the properties are identical no matter to level off. The size at <1500°C is 50 Å and increases which direction they are measured in. Many conven- to about 400 Å at 2000°C. tional graphites are anisotropic. This means the A difference will be noted in petroleum versus properties vary depending on which direction you pitch coke. The pitch coke does not increase to the test them in. The high degree of isotropy makes POCO same size as the at the same tempera- graphites useful in many applications where an aniso- ture. It parallels about 75 Å lower, beginning about tropic material would fail. It also allows for maximum 1700–1800°C. L is the basal plane size. L , which also utilization of material, as machining orientation is of a a increases, is the stacking direction height (Figure no importance. 1-5). The total size increases while the interlayer spacing, d, decreases. These changes, along with Temperature Effects processing parameters, account for the graphites’ There are two general types of carbon, those consid- excellent properties. ered to be “graphitizing” carbons and those that are “nongraphitizing.” The most significant difference is Density Effects found in the apparent layer size and apparent stack Isotropy is independent of density. A high- or low- height. For equal layer sizes, the apparent stack density material can be isotropic or anisotropic. height, i.e., average number of layers per stack, is less The general crystal “structure” is also independent for nongraphitizing carbons than graphitizing carbons. in that the greatest effects on density are due to pro- The layer stacking is more perfect in graphitizing car- cess parameters. The same crystal “structure” can bons than nongraphitizing. These apparent sizes and exist independent of the density of the bulk piece. heights are important in the first stages of carbonization.

1100°K Apparent Density 1500°K 1700°K Definition 2000°K The density of a substance is the amount of material, or mass, per unit volume. Density is ordinarily expressed in grams per cubic centimeter or pounds per cubic foot (1 g/cm3 = 62.4 lb/ft3). To determine the density of a specimen, one would first calculate its volume from the physical dimensions (for a rectangular solid, the volume is equal to the product of the length, width and thickness). Next, the mass would be determined by weighing the specimen. The density is determined by dividing the mass by the calculated volume.

Figure 1-9. A model of changes from mesophase to graphite during If the specimen were completely homogeneous, with heat treatment3 no flaws or voids, this method of determining density The structure of graphite with regard to interlayer would yield the theoretical value. Graphite materials spacings and crystallite size does change with temper- are, however, porous; hence, the term apparent ature (Figure 1-9). Interlayer spacing, d, decreases as density. heat-treat temperature increases. Beginning about In general, the differences in density of POCO graph- 1500°C, the interlayer spacing, d, decreases sharply ites reflect what some of the other physical properties from about 3.50 Å to about 3.40 Å by the time the tem- will be. The higher-density graphite will, generally, be perature reaches 2000°C. At this point it begins to stronger with a higher hardness value plus improve- level off, approaching 3.35 Å above 3000°C. The crys- ment in many other properties and characteristics. tallite size, La, increases as heat-treat temperatures

3 See footnote 1.

6 properties and characteristics of graphite Entegris, Inc. W D = V

V = l • w • t = 1.000 in • 0.5000APPARENT in • 0.5000 DENSITY in V = 0.25 in3

The mathematical expression for the determination of porous, the intrusion of water into the porosity is slow density is: and the accuracy with this method is ±1 percent if the submerged weight is taken quickly. W W 7.500 g D = D = = + 1.831 g/cm3 V The general stepsV in 4.097the “water cm3 method” are as follows: Where: D = Density in g/cm3 1. Support the piece of graphite by a thin wire/thread W W = Weight of specimen= in grams and weigh the piece of graphite in air. D 3 V = Volume of specimenV in cm 2. Submerge the piece in a container of water in Or, if the weight is expressed in pounds and the vol- such a way that the submerged weight can be ume is expressed in cubic feet, then the density would determined. W be in units of pounds perD cubic = foot. V = l • w • t = 1.000 in • 0.5000V in • 0.5000 in 3. Calculate the density by the following formula: V = 0.25 in3 W Sample Calculation: D = × DL W –W A graphite specimen has a length (l) of 1.000 inches, 1 2 V = l • w • t = 1.000 in • 0.5000 in • 0.5000 in a widthV (=w 0.25) and in thickness3 (t) of 0.500 inches, and Where: D = Density in g/cm3 a weight of 7.500 grams. Calculate the apparent W1 = Weight in air (in grams) density (D). W2 = Weight in water (in grams) D = Density of water V = l • w • t = 1.000 in • 0.5000 in • 0.5000 in L V = 0.25 in3 W 7.500 g D = = + 1.831 g/cm3 POCO Graphites vs. Conventional Graphites To convert to cmV3, multiply4.097 cm by3 16.387 (1 inch = Pr = –2γ cosθ POCO graphites are manufactured in a variety of 2.54 cm, Appendix A). grades covering the density range from 1.30 g/cm3 W 7.500 g 3 D = = + 1.831 g/cm3 to 1.88 g/cm . The density is a particularly important V 4.097 cm3 characteristic of graphite because, in addition to its inherent significance, it has a direct influence on Test Methods other properties. Generally, the physical and mechani- W 7.500 g D = = + 1.831 g/cm3 cal properties improve as theV maxdensity is increased; The standard methodV 4.097 commonly cm3 used to determine details will be presentedS = 0.0225 in later PdV sections. Commercial, the apparent density of graphiteW is described in polycrystalline graphites seldom exceed 80 percent of ® D = × D 0 ASTM Standard C559 and ResearchL & Development – 3 W1–W2 the theoretical density figure (2.26 g/cm ) due to voids Analytical Services Laboratory Instruction (TDI) and pores. Single crystal and pyrolytic graphites, (Appendix B). because of their highly ordered structures and absence W For premium graphites,D = such as× the DL POCO grades, an of pores, have closely approaching the theo- W1–W2 alternate method of determining apparent density is retical value. In comparison to most other materials of the “water method.” This is a method that can be used construction, graphite has a lowL density (Figure 2-1). This is a decided advantageC.S. = for the large majority of on objects of irregular shapeW where the volume would A be difficult to calculate.D = Even though× DL the graphite is applications. Pr =W –21–Wγ cos2 θ 25

20 Pr = –2γ cosθ L 3500 lbs C.S. = = = 14,000 psi 3 2 15 A 0.25 in V max S Pr= 0.0225= –2γ cos PdVθ 10 Density, g/cm Density, 0

V max 5 S = 0.0225 PdV

0 0 Rubber ABS/PVC *Graphite: Graphite: Aluminum Silicon Alumina Brass Copper Tungsten Gold Platinum V max = Poly- LPyrolitic Carbide Carbide S crystalline C.S.0.0225 = PdV A *synthetic/manufactured graphite Figure 2-1. Typical densities of various0 engineering materials


L 3500 lbs C.S. = = L = 14,000 psi A C.S.0.25 = in2 A

L 3500 lbs C.S. = = = 14,000 psi A 0.25 in2

L 3500 lbs C.S. = = = 14,000 psi A 0.25 in2 POROSITY

Temperature Effect pressure and volume filled, the pore size and pore volume can be determined. There are certain disad- The apparent density will be influenced by tempera- vantages of this method, such as: ture during the graphitization process. Generally, the higher the graphitization temperature, the higher the 1. The pores are not usually circular in cross-section density will become. There are other factors which and so the results can only be comparative. may contribute to this also, but there is an appreciable 2. The presence of “-bottle” pores or some other density increase as you go from 2000°C to 3000°C. shape with constricted “necks” opening into large void volumes. The pore radius calculated by the Washburn equation is not truly indicative of the Porosity true pore radius and capillaries are classified at too small a radius. Definition 3. The effect of compressibility of mercury with The standard definition for porosity, as found in ASTM increasing pressure. This should be corrected for Standard C709 which has definitions of terms relating by carrying out a blank run. to manufactured carbon and graphite, is “the percent- 4. The compressibility of the material under test. This age of the total volume of a material occupied by both is a problem of particular importance for materials open and closed pores.” When one calculates the which have pores that are not connected to the sur- apparent density of a material, the pore volume is face, e.g., cork. Additionally, pore walls may break included in the calculation. This results in typical under the pressures used if the material under test maximum densities for nonimpregnated manufactured is relatively weak. This could cause a bias in the 3 graphites of 1.90 g/cm . The theoretical density of data. graphite is 2.26 g/cm3. This means that in the very best case, about 16 percent of the volume of a bulk piece of 5. The assumption of a constant value for the surface graphite is open or closed pores. This porosity plays an tension of mercury. important role in many ways, as will be discussed later. 6. The assumption of a constant value for the angle of contact of mercury. The characteristics of the porosity of POCO fine- grained graphites have been studied extensively.4 30

25 Test Methods 20 There is no recognized ASTM standard for measuring the porosity of manufactured graphites at this time. 15

A number of techniques may be employed for the pur- 10 pose and are widely in use today. It is important to state the method by which porosity data is determined 5 because each method imparts its own bias. (% of Theoretical) Closed Porosity 0 One of the more widely used methods is mercury poro- 1.51.6 1.71.8 1.9 Apparent Density (g/cm3) simetry. Two other methods in use are gas absorption by the BET technique and direct image analysis of the Figure 3-1. Closed porosity vs. density microstructure. The latter is gaining increased accep- 1.1 tance as a means of measuring more accurately the 1.0 real pore structure. The advent of computer and video 0.9 equipment has pushed this technique to the forefront 0.8 of the porosimetry field. There are, nonetheless, limi- 0.7 tations to this method also. 0.6 0.5 re Diameter (µm) The mercury porosimetry technique is the method 0.4 used for the data reported in POCO graphite litera- 0.3 0.2 ture. It involves basically pushing, under increasing Average Po 0.1 pressure, mercury into the pores and as a function of 0.0 1.5 1.6 1.7 1.8 1.9 Apparent Density (g/cm3) 4 Brixius, W.H., Dagdigian, J.V., “Mercury Porosimetry Analysis of Fine- Grained Graphite," Conf. Proc., 16th Biennial Conference on Carbon, Figure 3-2. Pore diameter vs. density (1983), pp. 465–466.


V = l • w • t = 1.000 in • 0.5000 in • 0.5000 in V = 0.25 in3 V = l • w • t = 1.000 in • 0.5000 in • 0.5000 in EntegrisV =has 0.25 carried in3 out extensive analysis via mer- studies confirm this observation. Closed porosity was cury porosimetry to determine fundamental porosity also found to increase with graphite apparent density. parameters such as pore size and distribution, pore As can be predicted on the basis of the above surface volume and surface area. Using these pore characteris- area equation and the above observations, surface tics, two basic linear correlations were observed: area varies inversely with graphite apparent density. closed porosity versus graphite apparent density A greater amount of surface area is observed in the (Figure 3-1) andW average7.500 pore g diameter versus lower-density graphite than in the higher-density prod- D = = + 1.831 g/cm3 graphite apparentV density4.097 cm(Figure3 3-2). uct. At first glance, these observations are surprising W 7.500 g and may even seem contradictory. Why should closed The mercuryD =porosimetry = measurements+ 1.831 g/cm were3 made 4.097 cm3 porosity increase when a concomitant increase in the on a MicromeriticsV ® Mercury Porosimeter, Model 915- pore diameter is also observed? 2. A surface tension constant of 480 dynes/cm and a wetting contact angle of 140 degrees were assumed Although cause-and-effect relationships are difficult to and used in the Washburn equation5 where P is pres- establish, graphite porosity, pore size and surface area sure in psi, r is the pore radius in cm, γ is the surface are all physically related to density. Their relationship tension and θ is the contact angle. A pressure and to density, whether direct or inverse, has implications penetration volume readingW were derived from the on the structural properties of POCO graphite. The fol- = mercury porosimetryD apparatus.× DL lowing physical model is advanced to rationalize the W1–W2 W above relationships. Penetration volumeD and = pressure× DdataL were used to generate a printout of pressure,W1–W2 volume, pore size and One notes that as graphite density increases, closed percent porosity relationships. Graphical plots were porosity and average pore size also increase while generated to summarize pore size distribution infor- graphite surface area decreases. As the graphite struc- mation; percent porosity was plotted as a function of ture increases in density, the smaller pores can be pore diameter. imagined to become more and more occluded until Pr = –2γ cosθ they are isolated from the rest of the pore system. As this process occurs, the smaller-diameter pores are The surface area wasPr also = –2 determinedγ cosθ for each sample systematically eliminated until only the larger, less as described in the relationship6 where S is the surface complex pores remain. This also creates a larger area in square meters per gram. In addition, the total amount of closed porosity. Thus, not only does the closed porosity was determined from the theoretical average pore diameter increase as a result of the porosity and the observed (open) porosity. Percent elimination of small open pores, but pore surface open and closed porosity wereV max expressed as theoretical area is reduced, since only pores with less branched porosity. S = 0.0225 PdV structures remain.

V max0 S = 0.0225 PdV Mercury porosimetry data on fine-grained graphites reveals a definite relationship between closed porosity 0 and apparent density with the closed porosity increas- ing as the apparent density increases. The pore size The above porosity parameters are displayed in graphi- also increases as apparent density increases. cal form as closed porosity versus apparent density and average pore diameter versusL apparent density C.S. = POCO Graphite vs. Conventional Graphite and average pore diameter versusA apparent density in Figures 3-1 and 3-2, respectively.L Linear regression The pore volume will be the same for all graphite C.S. = analysis was performed to determineA the best fit equa- with the same apparent density, but that is where the tion (dotted lines represent limits at the 95 percent similarity ends. The pore diameter of POCO graphites confidence level). range from 0.2 microns nominal to 0.8 microns nomi- nal for our 1 and 5 micron grades and densities. The The results of these porosimetry studies indicate a lin- L 3500 lbs open porosity ranges from 75 percent open to 95 per- ear relationshipC.S. = between = average= pore 14,000 diameter psi and 2 cent open and the pores are generally spherical in graphite apparentA density.0.25 That in is, as the graphite L 3500 lbs shape. The smaller pore size materials have a very apparent densityC.S. = increases = the average= 14,000 size psi of the 2 large surface area associated with them. Conventional pores increases. PreviousA 0.25 in-house in photomicrograph graphites will, at best, have typical pore sizes only down to several microns in size. The distribution of 5 Washburn, E.W., Phys. Rev., 17, 273 (1921). pore sizes is very narrow for POCO graphites, while 6 Orr, C., Jr., “Application of Mercury Penetration to Materials they are generally broader for conventional graphites Analysis," Publication 9-AN-1 from Micromeritics Instrument Corporation, 8. (Figure 3-3).


100 10,000

90 POCO graphite PS-1 80 Conventional graphite 1000 70


50 100

Porosity (%) Porosity DFP-1 40 Helium Flow (cc/min) 30

20 10 SFG-1


0 0.01 0.1 110 1 Pore Diameter (microns) 20 40 60 80 100 120 140 160 Pressure (psi) Figure 3-3. Pore size distribution: POCO graphite vs. conventional All test samples: 1/4” thick graphite Flow cross section: 1.25” diameter

The fineness of the porosity allows Entegris materials Figure 3-4. Helium flow vs. pressure for some POCO graphite to be modified to create truly impermeable graphite. materials

With various post-processing techniques, Entegris can 5 seal, fill or close the porosity, depending on the end application. The high degree of open porosity also allows Entegris to purify the material to less than 5 parts-per-million (ppm) total impurities, by ash 4 analysis. Figure 3-4 shows helium flow data on various CZR-1 grades of POCO graphite. It clearly shows a wide vari- ety of capabilities for POCO graphites. The helium flow 3 test for checking the permeability, like the mercury ight Pick-up (%) porosimetry test, has its limits or bias and should be clearly identified when using data generated by it. 2 Permeability is simply the rate of flow of a medium such as a gas or liquid through a material while under Cumulative We 1 a pressure gradient. The pores of POCO graphite are DFP-1 not only uniformly distributed, but are well intercon- nected so flow can take place through them. There are SFG-1 applications (such as filters) where this is important. 0 024487296 120 144 168 192 Another feature of the small pore size is that some liq- Soak Time in Water (hours) uids will not generally penetrate the pores readily. For instance, it has been determined that after soaking in Figure 3-5. Water absorption of POCO graphites water for seven days, a sample of DFP-1 picked up less than one percent by weight of the water (Figure 3-5). This could be an advantage in some applications. However, in other applications, such as wanting to infiltrate a liquid such as molten copper, very high pressures are required at high temperatures to accom- plish it. This is a decided disadvantage.


Temperature Effects sense, cutting. Cutting would clearly include machin- ability as an index of hardness. This is much less As temperature increases, pores will expand along precise than conventional hardness testers, but with the rest of the matrix to the point of opening can be an indicator of relative usefulness. what were previously closed pores. As graphitization temperature increases, the pores will generally be found to be slightly smaller in size. It is uncertain Test Methods what would happen to an impermeable graphite at There are two standard test methods for hardness room temperature if it were raised to very high tem- of graphite: ASTM Standard C886, the Scleroscope peratures. Theoretically, if it is surface sealed, it may Hardness method, and C748, the Rockwell Hardness retain its impermeability. If it is densified, additional method. The Shore Scleroscope measures hardness pores may open and thus render it permeable once in terms of the elasticity of the material. A diamond- more. tipped hammer in a graduated glass tube is allowed to fall from a known height onto the specimen to be Density Effects tested, and the hardness number depends on the height to which the hammer rebounds; the harder Mercury porosimetry data on fine-grained graphites the material, the higher the rebound (see also Mohs reveals a definite relationship between closed porosity hardness). Brinell hardness is determined by forcing and apparent density with the closed porosity increas- a hardened steel or carbide ball of known diameter ing as the apparent density increases. The pore size under a known load into a surface and measuring the also increases as apparent density increases. The rela- diameter of the indentation with a microscope. The tionship is linear, with closed porosity increasing as Brinell hardness number is obtained by dividing the density increases and pore size also increasing as load, in kilograms, by the spherical area of the inden- density increases. If post-processing densification tation in square millimeters; this area is a function of techniques were employed to raise the density, the the ball diameter and the depth of the indentation. pore size and pore volume would decrease. The Rockwell hardness tester utilizes either a steel ball or a conical diamond known as a brale, and Hardness indicates hardness by determining the depth of penetration of the indenter under a known load. Definition This depth is relative to the position under a minor initial load; the corresponding hardness number is Although the term “hardness” has comparative signifi- indicated on a dial. For hardened steel, Rockwell cance in an engineering sense, it is generally not testers with brale indenters are particularly suitable; considered to be a fundamental property of matter. they are widely used in metalworking plants. The index of hardness is generally a manifestation of several related properties. In graphite, for instance, The Vickers hardness tester uses a square-based dia- the particle size and porosity, and apparent density mond pyramid indenter, and the hardness number is have an influence on the hardness value. The hard- equal to the load divided by the product of the lengths ness value can be changed by the graphitization of the diagonals of the square impression. Vickers temperature as well, which generally relates back to hardness is the most accurate for very hard materials a strength characteristic such as shear strength at the and can be used on thin sheets. crystallographic level. Different hardness testers are influenced by different properties and as a conse- The Scleroscope hardness test method is based on the quence cannot be correlated very well. Comparatively rebound height of a diamond-tipped hammer off the soft materials may be hard in the sense that they can sample’s surface after it falls a fixed distance. The resist abrasion stresses, whereas harder materials in scale is unitless, with the degree of hardness directly the sense of indentation hardness may fail completely related to the height of the rebound. The higher the under the same circumstances. It should be obvious rebound, the “harder” the material. There are many at this point that the term “hardness” is relative and factors that affect the reproducibility and accuracy of hardness data must be interpreted in relation to the the data. Studies at Entegris indicate that sample size, type of hardness tests used. i.e., mass, has a significant bearing on the results as well. The results of a large billet will generally be An appropriate definition of hardness then would be higher than a small test sample. the resistance of a material to permanent deformation of its surface. The deformation may be from scratch- The Rockwell hardness test method is based on the ing, mechanical wear, indentation or in a broader differential—the depth of indentation—produced on


a sample’s surface by a primary and secondary load however, a material such as copper, which is very soft and a specific-sized indenter. The hardness is taken and ductile, is introduced into the pores, the overall directly from a dial reading. The higher the number, hardness drops in direct relationship to the volume of the “harder” the material. A variety of loads and copper filling the pores. Hence, a material with 75 per- indenter sizes are available depending upon the cent open porosity will have a higher hardness when material being tested and its expected hardness. For filled with copper than a material initially having 95 graphite, the method specified calls for use of the “L” percent open porosity when it is filled with copper. scale with a 60 kg load and a 6.35 mm diameter steel- 84 ball indenter. 82 This works well for conventional graphites, but the ultra-fine particle POCO graphites are usually off the 80 scale on the high side when tested with this method. 78 Another scale and/or a smaller indenter would be 76 more appropriate, but a standard method has not been developed for use. However, a number of our custom- 74 ers specify use of the Rockwell 15T scale for hardness 72 data. This is a 15 kg load with a 1.5875 mm diameter steel-ball indenter. There are a number of factors 70 which affect the accuracy of this method, regardless 68

of the scale used. They are listed in TDI, Shore Scleroscope Hardness (SSH) Section 3.4 (Appendix B). 66 64

POCO Graphite vs. Conventional Graphites 62

Since hardness is influenced by a number of other fac- 60 tors, the comparison of POCO graphite to conventional 1.56 1.60 1.64 1.68 1.72 1.76 1.80 1.84 1.88 graphites is relative at best. If all factors were held Apparent Density (g/cm3) constant, including graphitization temperature and no Figure 4-1. Correlation of hardness to density in POCO graphites artificial densification, etc., based strictly on particle size alone, POCO graphite would have a higher hard- ness number. Compressive Strength Temperature Effects Definition Generally speaking, the higher the temperature, the softer or lower the hardness becomes. There is a direct The easiest strength characteristic of a material to relationship to graphitization temperature. The lower understand and measure is the compressive strength. this temperature is, the higher the hardness number When a material is positioned between two flat, paral- will be. As the temperature increases to about 3400°C, lel platens and a continually increasing compressive the hardness will continue to slowly drop. From room force is applied, either of two things can occur: temperature up to the material’s original graphitiza- 1. If the material is ductile, such as copper or iron, tion temperature, the hardness will show basically no atomic or molecular bonds can be re-formed easily; change. therefore, when crystalline planes begin to slip across each other, the atoms will readily re-form Density Effects bonds with other atoms. Consequently, it is quite There is a correlation of hardness to density for the possible to flatten a very ductile material, such as base graphite (Figure 4-1). As density increases, a gold, into a very thin sheet (if high enough compres- general increase is seen in hardness. This is associated sive load is applied). with the amount of porosity, which basically lowers the 2. If the material is brittle, such as graphite or many resistance to penetration. The lower the density, the materials, atomic or molecular bonds can- greater the pore volume and the less the resistance to not be re-formed easily; therefore, when crystalline the penetrator and, hence, a lower hardness number. planes begin to slip, catastrophic failure occurs and If the porosity is filled with a material for densification the material . purposes, the hardness might increase slightly. If,


W D = V

V = l • w • t = 1.000 in • 0.5000 in • 0.5000 in V = 0.25 in3

V = l • w • t = 1.000 in • 0.5000 in • 0.5000 in V = 0.25 in3

W 7.500 g D = = + 1.831 g/cm3 V 4.097 cm3

W 7.500 g D = = + 1.831 g/cm3 V 4.097 cm3

W D = × DL W1–W2

W D = × DL W1–W2


Pr = –2γ cosθ

V max The compressive strengthS = 0.0225 of a brittle PdV material is POCO Graphites vs. Conventional Graphites expressed as the maximum force0 per unit area that can be withstood before failure occurs. It is usually POCO graphites have high compressive strengths compared to conventional materials. The compressive expressed in pounds per square inch (lb/in2 or psi) or V max strengths of Entegris materials range from 69 MPa in megapascal (MPa)S = in 0.0225 metric PdVunits. The mathemati- cal expression for the determination of compressive (10,000 psi) to over 207 MPa (30,000 psi) depending strength is: 0 on the grade selected. These values are two to three times higher than most other graphites. The final L C.S. = heat-treating temperature used in the manufacture of A a carbon/graphite material has a marked effect on its Where: C.S. = Compressive strength compressive strength; the lower the final temperature, L = Load required to cause failure the higher the compressive strength. L A = Cross-sectionalC.S. = area of specimen A Temperature Effects Sample Calculation:L 3500 lbs As the temperature of a piece of graphite is increased, C.S. = = = 14,000 psi A graphite specimenA with0.25 a cross-sectionalin2 area of its compressive strength increases, up to about 2500°C. 0.25 square inches (in2) fails when a load of 3500 This is particularly important in such applications as pounds is applied; calculate the compressive strength. hot pressing dies, where the material is subjected to both high temperature and high stress levels. L 3500 lbs C.S. = = = 14,000 psi Depending on the grade and type of graphite, the A 0.25 in2 increase in strength will be from 15 to 50 percent To convert to MPa, multiply by 0.0068948. higher at 2500°C than it is at 25°C. Compressive strength = 97 MPa The graphitization temperature also has a marked effect on the compressive strength. The lower the NOTE: 1 psi = 0.0068948 MPa Other units and the appropriate conversion graphitization temperature, the less graphitic the factors are given in Appendix A. material is and the higher the compressive strength will be. This is readily seen when comparing carbon- Test Method based material to graphite-based material for areas such as mechanical application. The standard method of measuring the compressive strength of a graphite specimen is described in ASTM Density Effects Standard C695. Entegris’ method differs in that the specimen used is rectangular rather than a right cylin- As with many other properties, the compressive der. Cushion pads are not used either (TDI in strength of graphites changes with apparent density; Appendix B and Figure 5-1). the higher density having the highest strength (Figure 5-2).

Upper head 30 28 Spring attachment Spring attachment 26 Spherical bearing block Contact block 24 22 Test specimen Clear plastic safety shield 20 Contact block 18 Lower head 16 14

Compressive Strength (ksi) 12 10 Figure 5-1. Elements of compressive strength load train 8 6 1.56 1.60 1.64 1.68 1.72 1.76 1.80 1.84 1.88 Apparent Density (g/cm3)

Figure 5-2. Compressive strength of the graphite changes when apparent density increases


PL F.S. = 2 Flexural Strength Sample Calculation: W(T) Assume a graphite specimen 4.0 inches long by 0.75 Definition inches wide by 0.50 inches thick is flexure tested on a fixture with support roller pins 3.0 inches apart. The Flexural strength, tensile strength and compressive specimen fails under a 500 pound load; calculate its strength are the three common tests performed to flexural strength. measure the strength of materials. In flexural strength PL 500 × 3.0 lbs 1500 testing, a steadily increasing bending movement is F.S. = = = psi = 8000 psi 2 2 2 applied to a long bar until the material eventually W(T) 0.75 × (0.50) 0.1875 in ruptures. If the material is ductile (like copper), it To convert to MPa, multiply by 0.0068948. will bend prior to breaking. However, if the material is brittle (such as chalk or graphite), it will bend very Flexural strength = 55 MPa little before it fails catastrophically. Test Method Load 2000 lbs Flexural strength can be defined as the maximum Tensile strength = = = 8000 psi stress in bending that can be withstood by the outer The details of the procedureArea commonly0.25 in2 used for fibers of a specimen before rupturing (Figure 6-1). testing the flexural strength of graphite are found in ASTM Standard C651. Entegris’ method (TDI 1/ L 1/ L 3 P 3 in Appendix B) differs in that the specimen geometry has a 1:1 ratio in thickness and width rather than the 2:1 width to thickness. The fixture used is Upper span not exactly as described either, but with a surface finish of less than 32 micro inches Ra on the sample, Test specimen T T the frictional component = E is= minimized,Constant and results are comparable to thosee obtained with the fixture Support span Roller recommended by ASTM. Regardless of the procedure pin followed, steps must be taken to avoid factors that bias

L the results, such as improper sample alignment, rough and/or non-parallel surfaces. It is also important to know whether the reported strength values were Cross section of specimen P L obtained from three-pointT = or four-pointe = δ loading. A Lo T For example, a test specimen typically has a uniform rectangular cross-section but the load may be applied W in three or four point as illustrated in Figure 6-2. Note Figure 6-1. Four-point loading fixture the shaded areas indicating the stress distribution. In 3-point loading, the peak stress occurs on a single line at the surface of the testP bar/ A andPL opposite the point of The mathematical expression for calculating flexural E = = o strength is: loading. The stress decreasesδL / Lo linearlyδL along the length of the bar, and into the thickness of the bar, until PL F.S. = reaching zero at the bottom supports. W(T)2 Unlike the 3-point bend tests, where the peak stress Where: F.S. = Flexural strength in pounds per square inch occurs on a single line opposite the point of loading, (psi) 4-point loading distributes the peak stress over an P = Load in pounds at failure area that isPL odetermined(500 by lbs)(1) the width of the sample L = Length between outer support roller pins E = = = 1.6 × 106 psi and the spanδLA of the(0.008 top in)(0.038 loading insupports,2) respectively. in inches Observe how the tensile stress distribution decreases W = Width of specimen in inches PL 500 × 3.0 lbs 1500 linearly from the area of peak stress on the tensile F.S. =T = =Thickness of specimen= in inchespsi = 8000 psi W(T)2 0.75 × (0.50)2 0.1875 in2 face, and into the thickness of the bar, until reaching zero at the bottom span supports. The increased area and volume under peak tensile stress, or near the AR peak tensile stress, in 4-pointER = loading increases the probability of encountering a Llarger flaw.

Load 2000 lbs Tensile strength = = = 8000 psi Area 0.25 in2 (0.25 in2)(0.00425 ) ER = Ω 2.0 in 14 PROPERTIES AND CHARACTERISTICS OF GRAPHITE ENTEGRIS, INC.

T = E = Constant e

P L T = e = δ A Lo

P / A PL E = = o δL / Lo δL

PL (500 lbs)(1) E =o = = 1.6 × 106 psi δLA (0.008 in)(0.038 in2)


(0.25 in2)(0.00425 ) ER = Ω 2.0 in FLEXURAL STRENGTH

Load than 7 MPa (1000 psi) to around 41 MPa (6000 psi). The fine particle size and homogeneous structure of POCO graphites contribute to their high flexural 3-point Flexural Test strengths. Another important characteristic of this property in POCO graphites is that the flexural strength is the same for samples cut from any direc- tion or orientation of the parent block of material. This characteristic is called isotropy; POCO graphites are said to be isotropic, whereas most other graphites Peak stress are anisostropic. In conventional graphites that are molded or extruded, the ratio of flexural strengths between the “against grain” and “with grain” directions Load may range from 0.3 to 0.5. Temperature Effects 4-point Flexural Test One of the unusual properties of graphite is that it gets stronger as it gets hotter (up to about 2500°C). This is contrary to most materials, which lose strength as the temperature increases. The flexural strength of graphites will increase by 20 to 50 percent when the

Peak stress test temperature is increased from 25°C to 2500°C. Figure 6-2. 4-point bend strength <3-point bend strength Density Effects The probability of detecting the largest flaw in the Flexural strength, like many other physical properties specimen during 3-point loading is minimized since of graphite, increases with increasing density. For the largest flaw must be at the surface and along the example, nominal flexural strength of 5 micron line of peak stress. Consequently, the specimen frac- graphite (DFP-1, TRA-1 and CZR-1) ranges from tures at either a smaller flaw or within a region of 6000–17,000 psi for densities of 1.60 (g/cm2) and lower stress, thus yielding artificially higher strength 1.88 (g/cm2), respectively (Figure 6-3). One micron values in comparison with 4-point loading results. The or SFG exhibits the highest flexural strength indicat- strength limit of the material, or even the local stress ing 18,000 psi. and flaw size that caused fracture, is not revealed in 3-point loading. It only indicates the peak stress on 24 the tensile surface at the time of fracture for a given 22 material. 20 18 POCO Graphites vs. Conventional Graphites 16 In a brittle material such as graphite, the flexural 14 strength is particularly sensitive to flaws or defects 12 in the material. If a flaw is present within span L, i.e., between the outer support pins of the flexural 10 test specimen, then the load P required to break the Flexural Strength (ksi) 8 sample will be reduced. When failure occurs, it is cata- 6 strophic. The sample breaks suddenly and, frequently, 4 small chips and flakes of material break away at the 2 point of failure. 0 POCO graphites have flexural strengths covering the 1.56 1.60 1.64 1.68 1.72 1.76 1.80 1.84 1.88 3 range from 34 MPa (5000 psi) to over 124 MPa (18,000 Apparent Density (g/cm ) psi). These values are quite high when compared to Figure 6-3. Nominal flexural strength vs. apparent density of DFP-1, conventional graphites, which may range from less TRA-1 and CZR-1 graphites


Tensile Strength Test Method The details of the commonly used method for tensile Definition strength testing are shown in ASTM Standard C565. The tensile strength of a material can be defined as Other, more sophisticated testing equipment has been its strength when a pulling force is applied along the developed for this test; one of the better ones uses length of a sample. If a cylindrical bar of uniform hemispherical air bearings instead of chain connectors cross-section is subjected to a steadily increasing ten- to eliminate misalignment of the sample. sile (“pulling apart”) force along its axis, the material will eventually rupture and tear apart when a large POCO Graphites vs. Conventional Graphites enough force is applied. It is extremely difficult to The tensile strength of a brittle material, such as correctly determine the tensile strength of a brittle graphite, is very sensitive to defects or imperfections material, such as graphite. The reason for this is that in the material. The fine structure and uniformity of when brittle materials fail in tension, a crack origi- POCO graphites result in higher tensile strength for nates at some point and is rapidly propagated across Entegris materials as compared to conventional the specimen, causing catastrophic failure. Any imper- graphites. Typical tensile strength for conventional fection in the material, such as voids, or surface graphites ranges from 14 MPa (2000 psi) to 34 MPa scratches, will cause stress concentrations at that (5000 psi), as compared to 34 MPa (5000 psi) to 69 point. Since the stress at this point is higher than the MPa (10,000 psi) for Entegris materials (Table 7-1). overall average throughout the specimen, a crack will begin to propagate and premature fracture will occur. TABLE 7-1. TYPICAL ULTIMATE TENSILE STRENGTHS OF To alleviate this situation, carefully machined, highly VARIOUS MATERIALS polished specimens are used. Also, of no lesser impor- Ultimate Tensile Strengths tance is the alignment of the PLgripping apparatus on 3 F.S. = Material (10 psi) the specimen. Even small misalignmentsW(T)2 could cause premature fracture, resulting in an erroneous (low) Natural rubber >4 value for the tensile strength. ABS/PVC plastic 3–6 Tensile strength, like compressive strength, is Graphite (polycrystalline) 2–10 expressed in pounds per square inch and is calcu- Aluminum (wrought) 13–98 lated in the same manner as compressive strength, 3–20 i.e., the applied force at failure is divided by the cross-sectionalPL 500 area × of3.0 the lbs sample.1500 Alumina (ceramic) 20–30 F.S. = = = psi = 8000 psi W(T)2 0.75 × (0.50)2 0.1875 in2 Steel (wrought) 90–290 Sample Calculation: Brass 34–129 If a rod with a cross-sectional area of 0.25 in2 breaks Copper 29–76 at a load of 2000 pounds, then the tensile strength is Nickel 50–290 8000 psi. Gold 19–32 Load 2000 lbs Tensile strength = = = 8000 psi Area 0.25 in2 Platinum 18–30

To convert to MPa, multiply by 0.0068948. Tensile strength = 55 MPa

T = E = Constant e

P L T = e = δ A Lo


P / A PL E = = o δL / Lo δL

PL (500 lbs)(1) E =o = = 1.6 × 106 psi δLA (0.008 in)(0.038 in2)


(0.25 in2)(0.00425 ) ER = Ω 2.0 in TENSILE STRENGTH

Temperature Effects 9000 As the testing temperature of graphite is increased, its tensile strength increases; this is in sharp contrast to the behavior of metals, which show a decrease in 8000 strength as temperature increases. In an inert atmo- sphere (to avoid oxidation), the tensile strength of POCO DFP-1 graphite is almost 138 MPa (20,000 psi) at 5000°F (2760°C), as compared to 63 MPa (9150 psi) 7000 at room temp erature (Figure 7-1). This dramatic change is characteristic of graphite materials, even nsile Strength (psi) Te though most grades do not show the 100 percent increase that POCO graphites exhibit. 6000 The final heat-treat temperature of the graphite has a marked effect on its room temperature tensile strength: the lower the heat-treat temperature, the 5000 higher the strength. This is similar to the effect seen 2000 2200 2400 2600 2800 3000 3200 on other strength characteristics where the less Graphitization Temperature (°C) graphitic materials are harder and stronger. The Figure 7-2. Graphitization temperature effect on tensile strength relationship between graphitization temperature and tensile strength is shown in Figure 7-2. Density Effects 20 The tensile strength of graphite has a strong correla-

18 tion with density: as the density increases, the tensile strength increases. This is typical of the other strength 16 characteristics of graphites. Figure 7-3 shows the gen- eral relationship between density and tensile strength 14 for the DFP-1, TRA-1 and CZR-1 graphites. The nomi-

12 nal values of tensile strength range from 34 MPa (5000 psi) at 1.60 g/cm3 to 55 MPa (8000 psi) at 1.84 g/cm3. 10 14,000 nsile Strength (ksi) 8 12,000

Ultimate Te 6 10,000 4

2 8000

0 6000

01000 2000 3000 4000 nsile Strength (psi)

Temperature (°C) Te 4000 Figure 7-1. Ultimate tensile strength of DFP-1 2000

0 1.60 1.64 1.68 1.72 1.76 1.80 1.84 1.88 Apparent Density (g/cm3)

Figure 7-3. Nominal tensile strength vs. apparent density of POCO DFP-1, TRA-1 and CZR-1 graphites


PL 500 × 3.0 lbs 1500 F.S. = = = psi = 8000 psi W(T)PL 2 0.75500 × 3.0(0.50) lbs2 0.18751500 in2 F.S. = = = psi = 8000 psi 2 2 2 W(T)PL 0.75500 × 3.0(0.50) lbs 0.18751500 in F.S. = = = psi = 8000 psi W(T)2 0.75 × (0.50)2 0.1875 in2 PL 500 × 3.0 lbs 1500 F.S. = = = psi = 8000 psi W(T)2 0.75 × (0.50)2 0.1875 in2

Load 2000 lbs Tensile strength = = = 8000 psi Ash (ppm) = C – A × 1,000,000 = LoadArea 20000.25 inlbs2 Tensile strength = = = 8000 psi MODULUS OF BELASTICITY – A 2 LoadArea 20000.25 inlbs Tensile strength = = = 8000 psi 35.0004 – 35.000 Area 0.25 in2 × 1,000,000 = 5 ±1 ppm Load 2000 lbs 115.0000 – 35.000 Tensile strength = = = 8000 psi Area 0.25 in2

T Modulus of Elasticity = E = Constant eT = E = Constant C–A 100 Where: T = Stress eT Definition = E = Constant B–A e = Strain e The elasticity of a material is related to a uniformly T E = Modulus of =elasticity, E = Constant or Young’s modulus increasing separation between the atoms of that mate- e rial. As a consequence, the elasticity is directly related Stress is the load per unit area and strain is the ratio to the bonding between atoms and the respective of change in length to theP original Llength, i.e., T = e = δ energies associated therewith.In K α E/RT This can be readily AP LL T = e = δo demonstrated by showing the general relationship A L P δLo between modulus of elasticity (MOE) and melting T = e = A L points of various materials. The higher the melting Where: P = Load P δLo A = Area T = e = point (i.e., the energy required to disrupt the atom- A Lo to-atom bonds), the higher the modulus of elasticity δL = Change in length (L – Lo) WTO–WTF 10.0000–9.9900 Lo = Original length %WT(FigureL = 8-1). The ×presence 100 = of a second phase× 100 of = dif 0.1%- P / A PL WT 10.0000 E = = o fering modulusO results in most of the stress being PL // AL PLL Therefore: E =δ o = δ o carried by the higher modulus phase. Porosity, which δPL // ALo PLδL is uniformly distributed and continuous, constitutes a E = = o L / L L δP / Ao PLδ second phase. The effect on the modulus in materials E = = o of less than 50 percent pore volume can be repre- δL / Lo δL sented by the relationship: Sample Calculation: 2 If a tensile sample with a diameter of 0.220 inch and a Eo (1 – 1.9P + 0.9P ) PL (500 lbs)(1) E =o = = 1.6 × 106 psi gage length of 1.000 inch breaks at2 500 pounds with a δPLLAo (0.008(500 in)(0.038 lbs)(1) in ) Where: P = Porosity (volume fraction) gage lengthE = increase = of 0.008 inch, the= 1.6 tensile × 106 MOE psi LA (0.008 in)(0.038 in2) δPL 6 (500 lbs)(1) Eo = Original modulus of elasticity wouldE be = 1.6o × = 10 psi. = 1.6 × 106 psi LA (0.008 in)(0.038 in2) δPL (500 lbs)(1) Another way of expressing the modulus of elasticity is E =o = = 1.6 × 106 psi by referring to Hooke’s law, which applies to materials δLA (0.008 in)(0.038 in2) below their elastic limit. Basically, Hooke’s law says The modulus of elasticity is usuallyAR expressed in that the average stress is proportional to the average ER = millions of pounds per squareAR Linch (106 psi), or in strain. ER = N/mm2 in metric units. ARL 70 ER = L AR Test Method ER = 60 W (0.25 in2)(0.00425L ) The standard methodER = of measuring modulusΩ of elastic- (0.25 in22.0)(0.00425 in ) Single ity for graphiteER when = not determined fromΩ samples Al2O3 crystal 50 graphite tested in tension is described(0.25 in22.0)(0.00425 in in ASTM Standards) C747 ER = Ω Mo and C769. The methods are 2.0approximations in derived TiC (0.25 in2)(0.00425 Ω) 40 from other properties.ER = A more accurate, but also more

psi) 2.0 in 6 difficult, means is to attach proper strain measuring M O gages to tensile strength samples and measure strain 30 g MOE (10 along with stress during a tensile test. Then, employ- ing Hooke’s law, the modulus can be calculated. 20 Ca Pt POCO Graphites vs. Conventional Graphites Al 10 The modulus of elasticity varies for the many different Phenolic SiO2 glass Polycrystalline plastic graphite grades of graphite available; therefore, a significant NaCl 0 difference in the modulus for POCO materials com- 0 500 1000 1500 2000 2500 3000 3500 4000 pared to conventional graphite is not seen (Table 8-1). Melting Point (°C) The most notable difference, however, is the isotropy Figure 8-1. Modulus for elasticity vs. melting point for various of POCO which assures a modulus of the same value materials in any direction as compared to many conventional graphites being anisotropic.


PL 500 × 3.0 lbs PL 1500 F.S. = = F.S. = = psi = 8000 psi W(T)2 0.75 × (0.50)2 W(T)0.18752 in2

Load 2000 lbs Tensile strength = = = 8000 psi 2 PL 500 × 3.0Area lbs 0.251500 in F.S. = = = psi = 8000 psi W(T)2 0.75 × (0.50)2 0.1875 in2

LoadELECTRICAL2000 lbs RESISTIVITY Tensile strength =T = = 8000 psi =Area E = Constant0.25 in2 e

TABLE 8-1. MODULUS OF ELASTICITY OF Density Effects VARIOUS GRAPHITES P L The density relationshipT = is evidente = δ with about a Apparent MOE 15 to 20 percent increaseT A in modulusLo detected as Material Density (g/cm3) (106 psi) = E = Constant the density increasese from 1.65 g/cm3 to 1.77 g/cm3. POCO CZR 1.65 1.3 This follows the same pattern as other strength TRA 1.72 1.5 properties for graphite. DFP 1.77 1.6 P / A PL HPD 1.76 1.7 Electrical ResistivityE = = o δPL / Lo δLL Mersen® 2161 1.66 1.3 T = e = δ A L 2020 1.77 1.3 Definition o 2080 1.87 1.8 The electrical resistivity is that property of a material which determines its resistance to the flow of an Morgan P3W 1.60 1.4 electrical current and is an intrinsic property. The L-56 1.63 .5 electrical resistance of a substance is directly propor- PL (500 lbs)(1) P5 1.72 2.4 tionalE to = theo length = of the current path,= 1.6 i.e., × 10 as6 psithe P / A PL2 o current pathδLA increases,(0.008E = in)(0.038 the resistance = in ) increases. It is P03 1 0.82 1.8 δL / L δL also inversely proportional too its cross-sectional area, SGL Group, H-440 1.75 1.5 i.e., as the area increases, the resistance decreases. The Carbon HLM 1.75 1.8 Company® The mathematical expression for the determination H-463 1.75 1.8 of electrical resistivity is: AR H478 1.88 2.2 ER = Toyo Tanso SEM5 1.80 2.4 PL (500 lbs)(1)L E =o = = 1.6 × 106 psi USA® 2 SEM3 1.85 1.4 Where: ERδ =LA Electrical(0.008 resistivity in)(0.038 at in room) temperature A = Cross-sectional area (square inches) UCAR® AGSR 1.58 1.6 R = Electrical resistance of the material (ohms) (0.25 in2)(0.00425 Ω) CBN 1.67 1.8 L = DistanceER = between potential contacts (inches) 2.0 in CS 1.70 2.0 WG* Sample Calculation: 1.1 AG* AR For a graphite sample withER = a cross-sectional area of * Shows typical effects L 0.25 in2, distance between the potential contacts of Temperature Effects 2.0 inches and an electrical resistance reading of 0.00425 Ω (ohms); calculate the electrical resistivity. As with the other strength properties of graphite, as (0.25 in2)(0.00425 Ω) temperature increases, the modulus of elasticity also ER = 2.0 in increases, to a point beyond which it begins to drop rapidly. This is typically around 2500°C. The increase ER = 0.000531 Ω-in can be as much as 25 percent higher before the drop ER = 531 µΩ-in begins. The final graphitization temperature also has To convert to µΩ-cm, multiply by 2.54 an effect, but is relatively small. Electrical resistivity = 1349 µΩ-cm

Test Method The standard method of measuring the electrical resistivity of a graphite sample is described in ASTM Standard C611. At Entegris only two resistance read- ings are taken, using a special sample holder with eight electrical contacts which takes the equivalent of four readings at once. Entegris uses test method TDI (Appendix B).


POCO Graphite vs. Conventional Graphites 700 POCO graphites have electrical resistivity values that 650 fall into the range of most conventional graphites. POCO graphites have a range from around 450 µΩ-in to 1050 µΩ-in. This may be compared to copper which 600 has a range of 1.1 to 1.5 µΩ-in or tool which range from 7.1 to 7.5 µΩ-in. See Table 9-1 for other 550 common material resistivities. 500 TABLE 9-1. TYPICAL ELECTRICAL RESISTIVITY OF VARIOUS


Electrical Electrical Resistivity (µ Ω− in) Resistivity 400 Material** Range (µΩ-in) Comments TRA-1 DFP-1 POCO graphites 450–1000 Polycrystalline graphite 350 Toyo Carbon 300–600 Polycrystalline graphite graphites 300 Mersen graphites 550–1200 Polycrystalline graphite 0 500 1000 1500 2000 2500 Temperature (°C) Toyo Tanso USA 400–500 Polycrystalline graphite graphites Figure 9-1. Electrical resistivity vs. temperature of POCO graphite Copper 1.1–1.5 Pure, wrought Graphitization temperature also has an effect on Gold 0.9 Pure electrical resistivity. The higher the graphitization temperature, the lower the electrical resistivity Silver 0.6 Pure becomes, as measured at room temperature. Tungsten 2.2 Pure Carbon steels 7.1–7.5 Hardening grades, Density Effects wrought Density is a particularly important characteristic of 15.7–28.3 400 series, wrought graphite because, in addition to its inherent signifi- Cobalt base 36.6–74.3 Wrought cance, it has a large and direct influence on other superalloys properties. As the density of POCO graphite increases, Nickel base 3.0–52.4 Wrought and cast its electrical resistivity decreases (Figure 9-2). superalloys

Silicon 6 × 106 Semiconductor 1200 6 Silicon carbide 4 × 10 Semiconductor 1100 20 Silicon nitride 4 × 10 Insulator 1000 21 Alumina >4 × 10 Insulator in) 900 Ω− * Measured at room temperature in accordance with ASTM Standard 800 C611 and TDI 700 ** Select samples from each manufacturer but not all-inclusive 600

Temperature Effects 500

7 Electrical Resistivity (µ Electrical resistivity varies with temperature 400 (Figure 9-1). As the temperature increases from 300 room temperature to about 700°C, the electrical resis- tivity decreases. From that point, however, as the 200 temperature increases, the resistivity also increases. 1.601.561.64 1.68 1.72 1.76 1.80 1.84 1.88 Apparent Density (g/cm3)

Figure 9-2. Nominal electrical resistivity of DFP-1, TRA-1 and CZR-1 graphites vs. apparent density 7 Taylor, R.E. and Groot, H., Thermophysical Properties of POCO Graphite. (West Lafayette, Indiana: Purdue University, July 1978. [NTIS No. ADA060419]), p.16.


Thermal Expansion Test Method The standard method of measuring the coefficient of Definition thermal expansion of a graphite sample is similar to ASTM Standard E228, but there is no specific method The physical dimensions of a body are determined by for graphite. POCO has a horizontal silica (orton) the number and spacing of its atoms. At temperatures dilatometer and follows TDI (Appendix B). above 0 K, the atoms are always in constant vibration about their positions in the lattice. If energy is added to the material (by heating it), the atoms vibrate more POCO Graphites vs. Conventional Graphites vigorously, causing the macroscopic dimensions of the POCO graphites have high coefficients of thermal material to increase. expansion compared to conventional materials (Table 10-1). The coefficient of thermal expansion of POCO The coefficient of thermal expansion (CTE) is defined materials range from 7.0 × 10-6 to 9.0 × 10-6 (in/in)/°C, as the change in length of a substance per unit length depending on the grade selected. These values are two for a specific change in temperature. If this thermal to four times higher than most other graphites. expansion is hindered in any way, internal stresses will occur. TABLE 10-1. COEFFICIENT OF THERMAL EXPANSION When two different materials are to be joined 10-6 (in/in)/°C permanently, as in the electrical connection to an Material High Low incandescent lamp or a tube, it is important that they have nearly the same values of CTE if there Aluminum alloys (68–212°F) 24.1 22.3 is to be no danger of failure from cracking. The same 300 Stainless steels (32–212°F) 18.7 14.9 precaution applies to coatings or cladding of one Copper (68–572°F) 17.6 16.7 material to another. Nickel base superalloys (70–200°F) 17.8 10.6 The CTE is normally expressed as inch per inch per °C Cobalt base superalloys (70–1800°F) 17.1 16.2 or as inch per inch per °F, with the temperature range over which the CTE is applicable being specified. This nitride (70–1800°F) 7.5 – is done because, for some materials, the CTE will Titanium carbide (77–1472°F) 7.4 6.7 change with temperature. Tungsten carbide 7.4 4.5 The mathematical expression for the determination of Silicon carbide (0–2550°F) 4.3 3.9 the coefficient of thermal expansion is: Silicon nitride (70–1800°F) 2.5 – L CTE = δ POCO graphite (70–1832°F) 8.8 7.0 L(T2–T1) Where: CTE = Coefficient of thermal expansion δL = Change in sample length from the lower temperature to the upper temperature L = Sample length at the lower temperature 2.0164 in – 2.0000 in T1 CTE= Lower = temperature T2 = Upper2.0000 temperature in (1000°C seen – by23°C) sample

L Sample Calculation:CTE = δ L(T –T ) A two-inch graphite sample is2 heated1 from room tem- perature to 1000°C. The length uniformly increases until it is 2.0164 inches Q X in length at 1000°C; calculateK = ∂ the CTE. A∂T 2.0164 in – 2.0000 in CTE = 2.0000 in (1000°C – 23°C)

CTE = 8.39 × 10-6 (in/in)/°C To convert to (in/in)/°F: divide by 1.8 CTE = 4.65 × 10-6 (in/in)/°F

Q∂X (1000 Btu)(0.1 ft) K = = Q∂X A∂T (hr)(1K = ft2)(1036°F – 1018°F) ENTEGRIS, INC. A∂T PROPERTIES AND CHARACTERISTICS OF GRAPHITE 21 (1000 Btu)(0.1 ft) Btu-ft K = = 55.5 (hr)(1 ft2)(18°F) hr/ft2 °F

Q X (1000 Btu)(0.1 ft) K =∂ = A∂T (hr)(1 ft2)(1036°F – 1018°F)

(1000 Btu)(0.1 ft) Btu-ft K = = 55.5 2 2 (hr)(1 ft )(18°F)K = αCPρ hr/ft °F

(1–Ø) R = σƒ αE

K = αCPρ

(1–Ø)K R' = σƒ αE

(1–Ø) R = σƒ αE

KT R = S αET (1–Ø)K R' = σƒ αE 0.29 cal × cm 703 kg KT cm2 × sec × °C cm2 R =S = -6 -6 αET 8.4 × 10 0.112 × 10 kg °C cm2

KTS cal R = 216.7R = αcmET × sec

0.29 cal × cm 703 kg KT cm2 × sec × °C cm2 R =S = -6 -6 αET 8.4 × 10 0.112 × 10 kg °C cm2 cal R = 216.7 cm × sec THERMAL CONDUCTIVITY

Temperature Effects Thermal Conductivity As the temperature of a piece of graphite is increased, its coefficient of thermal expansion will become Definition greater (Figure 10-1). There is limited data to The thermal conductivity of a material is a measure describe its expansion characteristics above 1000°C, of its ability to conduct heat. A high thermal conduc- but the data available indicates a linear increase in tivity denotes a good heat conductor, while a low expansion as high as 2500°C. thermal conductivity indicates a good thermal 10 insulator (Table 11-1).

/°C) 9 -6 8 TRA-1 TABLE 11-1. TYPICAL THERMAL CONDUCTIVITY OF Taylor/Groot 7 VARIOUS MATERIALS 6 Thermal Conductivity Range 2 5 Material Btu-ft/hr/ft °F 4 Natural rubber 0.082 3 Nickel 6–50 2 Steel (wrought) 8–21 1 Coefficient of Thermal Expansion (10 0 Silicon carbide 9–25 0 500 1000 1500 2000 2500 Tungsten carbide 16–51 Temperature (°C) Brass 15–135 Figure 10-1. CTE vs. temperature of POCO TRA-1 graphite Iron (cast) 25–30 δL Density Effects POCO graphite CTE = 40–70 L(T2–T1) As with many other properties, the coefficient of ther- Platinum 42 mal expansion changes with apparent density; as the Conventional graphite 65–95 density of the material increases, so will the coeffi- Aluminum 67–135 cient of thermal expansion. Tungsten 97 2.0164 in – 2.0000 in Since the higher-density material has less porosity, Copper CTE = 112–226 there is less distance for the crystals to move unob- 2.0000 in (1000°C – 23°C) structed as the temperature is increased; therefore, Gold 172 higher density means greater expansion (Figure 10-2). Silver 242

8.5 The mathematical expression for thermal conductivity 8.4 (K) is: Q∂X 8.3 K = A∂T in/in/°C) -6 8.2 Where: Q = Rate of heat flow through a slab in Btu per hour 8.1 A = Cross-sectional area of the slab in feet2 ∂X = Thickness of slab in feet 8.0 ∂T = Temperature drop across the slab (°F) 7.9 Therefore, K is expressed as: Q∂X (1000 Btu)(0.1 ft) K = = Btu-ft/hr/ft2 °F 7.8 A∂T (hr)(1 ft2)(1036°F – 1018°F) or Coefficient of Thermal Expansion (10 7.7 (1000 cal/(meterBtu)(0.1 ft) °C sec) Btu-ft K = = 55.5 (hr)(1 ft2)(18°F)or hr/ft2 °F 7.6 W/m-K 1.601.561.64 1.68 1.72 1.76 1.80 1.84 1.88 Apparent Density (g/cm3)

Figure 10-2. CTE vs. apparent density of POCO DFP-1, TRA-1 and CZR-1 graphite


K = αCPρ

(1–Ø) R = σƒ αE

(1–Ø)K R' = σƒ αE

KT R = S αET

0.29 cal × cm 703 kg KT cm2 × sec × °C cm2 R =S = -6 -6 αET 8.4 × 10 0.112 × 10 kg °C cm2 cal R = 216.7 cm × sec L CTE = δ L(T2–T1)

2.0164 in – 2.0000 in CTE = 2.0000 in (1000°C – 23°C) THERMALL CONDUCTIVITY CTE = δ L(T2–T1)

Q X K = ∂ 2.0164 in – 2.0000 in Sample Calculation: A∂T CTE = IR sensor 2.0000 in (1000°C – 23°C) Assume a flat plate of graphite 0.1 foot thick has a sur- face area of one square foot. Heat is flowing through GE lens this plate at a rate of 1000 Btu per hour; the hotter Iris diaphragm surface is at a temperature of 1036°F, while the cooler surface is at 1018°F; calculate the thermal Cooling water conductivity. Graphite heater Q X (1000 Btu)(0.1 ft) Q∂X K =∂ = K = A∂T (hr)(1 ft2)(1036°F – 1018°F) Sample holder A∂T Vacuum seal (1000 Btu)(0.1 ft) Btu-ft K = = 55.5 (hr)(1 ft2)(18°F) hr/ft2 °F Sample holder adjustment

To convert to metric system of units, multiply by 0.413 Enlargement optic to obtain:

K = 22.9 cal/(meter °C sec) Decoupling mirror Q X (1000 Btu)(0.1 ft) K =∂ = Shutter To convert to SI system of units, multiply by 1.73 to A∂T (hr)(1 ft2)(1036°F – 1018°F) obtain: Adjustment laser Resonator (1000 Btu)(0.1 ft) Btu-ft K = 96.0 W/m-K K = = 55.5 (hr)(1 ft2)(18°F) hr/ft2 °F Rear mirror K = αCPρ Test Method Historically, two different techniques were employed Figure 11-1. Netzch LFA-427 for determining thermal conductivity over the of sample preparation. With less material require- temperature range of 110–3300 K. They are: ments and fast measurements, one typically obtains accuracy within ±5% when implementing careful 1. A comparative rod apparatusσ (1–Ø) from 110 K to 1250 K R = ƒ measurement techniques. Thermal conductivity is E 2. A radial inflow apparatus αfrom 1250 K to 3300 K calculated from the relationship: There is no standard method associated with graphite K = αCPρ for measurement of thermal conductivity. Where: K = Thermal conductivity (W/cm K) However, ASTM Standard C714-72 is routinely α = Thermal diffusivity (cm2/sec) used for determining thermal diffusivity and the CP = Heat capacity at constant pressure (J/g K) σƒ(1–Ø)K corresponding resultsR' are = used to calculate thermal ρ = Apparent density (g/cm3) αE conductivity. POCO graphite deviates from ASTM (1–Ø) Standard C714-72 in that this standard requires the POCO Graphite vs.R =Conventionalσƒ Graphites αE use of a flash lamp for sample heating and thermocou- From studies on POCO graphite and experimental ples for monitoring temperature. A similar technique work on other graphites, it is believed that the con- ® employed by Entegris utilizes a Netzsch LFA-427 ductivity of most graphites is mainly governed by (Laser Flash Apparatus), whichKT has a high-intensity Umklapp type -phonon interactions. laser to heat the sampleR surface= S and the resultant αE T For POCO graphite and most(1–Ø)K other graphites, thermal temperature change is monitored with an infrared R' = σƒ detector (Figure 11-1). conductivity begins to decreaseαE around 27°C (70°F) and continues to decrease as the temperature Thermal diffusivity measurements involve quantifying 0.29 cal × cm 703 kg increases to 3227°C (5800°F). Thermal diffusivity the rate of diffusion of heat through a solid where and thermal conductivity results for POCO graphite KT cm2 × sec × °C cm2 local temperatureR =S = and temperature gradients vary grades are indicated in Figures 11-2 and 11-3, respec- αE 8.4 × 10-6 0.112 × 10-6 kg with time. ThisT is a decided advantage in that thermal tively, as a function of temperature from 23–1650°C. diffusivity measurements°C are nonsteady-state,cm2 due to KT the transient nature of diffusion, thus eliminating It has been reported thatR = aboveS 2727°C (4900°F), cal αE the need for determiningR = 216.7 heat flux and maintaining electrons contribute about 20 Tpercent to the total cm × sec steady-state conditions. Nonetheless, close control thermal conductivity of these graphites. For more of local time-temperature relationships in the system highly graphitized graphites and other feedstocks, the results can be different. is required. Thermal diffusivity methods are also pre- 0.29 cal × cm 703 kg ferred over heat flux measurements due to the ease KT cm2 × sec × °C cm2 R =S = -6 -6 αET 8.4 × 10 0.112 × 10 kg °C cm2 ENTEGRIS, INC. PROPERTIES AND CHARACTERISTICS OF GRAPHITE 23 cal R = 216.7 cm × sec L CTE = δ L(T2–T1)

2.0164 in – 2.0000 in CTE = 2.0000 in (1000°C – 23°C) L CTE = δ L(T2–T1)

L CTE = δ L(T –T ) Q 2X 1 K = ∂ 2.0164 inA∂ –T 2.0000 in CTE = 2.0000 in (1000°C – 23°C)

2.0164 in – 2.0000 in CTE = 2.0000 in (1000°C – 23°C)

THERMAL SHOCK Q∂X (1000Q X Btu)(0.1 ft) K = = K = ∂ A∂T (hr)(1 ftA2∂)(1036°FT – 1018°F) (1000 Btu)(0.1 ft) Btu-ft K = = 55.5 (hr)(1 ft2)(18°F)Q X hr/ft2 °F K = ∂ A∂T


SFG-1 Thermal Shock DFP-1 1.0 PLS-1 Q X (1000 Btu)(0.1 ft) DefinitionK =∂ = A∂T (hr)(1 ft2)(1036°F – 1018°F) The ability of a material to be subjected to sudden 0.8

/sec) thermal gradients without weakening or fracturing is 2 (1000 Btu)(0.1 ft) Btu-ft K = = 55.5 referred to as Qits(hr)(1∂X thermal ft2)(18°F) (1000shock Btu)(0.1 resistance.hr/ft ft) 2 °FThe excel- K = = K = αCPρ 0.6 lent resistanceA ∂ofT graphite(hr)(1 toft2 )(1036°Fthermal – shock 1018°F) is due to a unique set of properties. High strength, low modulus (1000 Btu)(0.1 ft) Btu-ft of elasticityK = and low coefficient= of 55.5 thermal expansion 0.4 are the properties(hr)(1 that ft2)(18°F) are most important.hr/ft2 °FThis theo-

Thermal Diffusivity (cm retical relationship can be expressed as:8 0.2 (1–Ø) R = σƒ αE 0.0 0 200 400 600 800 1000 1200 1400 1600 1800 Where: R = Thermal shock resistance K = αCPρ Temperature (°C) σƒ = Fracture stress Ø = Poisson’s ratio α = Coefficient of thermal expansion Figure 11-2. Thermal diffusivity of POCO graphites E = Modulus of elasticityσƒ(1–Ø)K R' K= = αCPρ 140 αE This relationship applies only when the quench is so SFG-1 fast that the surface temperatureσƒ(1–Ø) reaches final value DFP-1 R = 120 PLS-1 before the average temperatureαE changes. For conditions when the heating rate is not very 100 high, a second relationship (1–Ø)can be established which = σƒ R KT includes thermal conductivity.R = αES This is expressed as: 80 αET (1–Ø)K R' = σƒ αE 60

Thermal Conductivity (W/m-K) Where: R' = Thermal0.29 shock cal ×resistance cm 703 kg (1–Ø)K =KT Fracturecm 2R'stress × = secσ ƒ× °C cm2 40 R σ=ƒ S = -6αE -6 Ø =α EPoisson’sT 8.4 ratio × 10 0.112 × 10 kg K = Thermal conductivity°C cm2 20 α = Coefficient of thermalKT expansion R = S 0 200 400 600 800 1000 1200 1400 1600 1800 E = Modulus of elasticitycal R = 216.7αET Temperature (°C) cm × sec Another way to express this would be: Figure 11-3. Thermal conductivity of POCO graphites KT R = S Density Effects 0.29 cal × αcmET 703 kg KT cm2 × sec × °C cm2 Where:R R= = ThermalS = shock resistance The density of graphite is significant in its effect, as on -6 -6 αET 8.4 × 10 0.112 × 10 kg the other properties. As the density of POCO graphite K = Thermal conductivity T = Tensile strength°C cm2 increases, its thermal conductivity also increases. S 0.29 cal × cm 703 kg = Coefficient of thermal expansion α KT cm2 × sec × °Ccal cm2 R =S = R = 216.7 ET = Tensile modulus -6of elasticity -6 αET 8.4 × 10 cm × sec0.112 × 10 kg This expression may be °Ca simpler form cmto 2use and consequently will be used in the sample calculation. cal R = 216.7 cm × sec

8 Kingery, W.D., Journal of the American Ceramic Society, 38 (1955), p. 3.


2.0164 in – 2.0000 in CTE = 2.0000 in (1000°C – 23°C)

Q X K = ∂ A∂T

Q X (1000 Btu)(0.1 ft) K =∂ = A∂T (hr)(1 ft2)(1036°F – 1018°F)

(1000 Btu)(0.1 ft) Btu-ft K = = 55.5 (hr)(1 ft2)(18°F) hr/ft2 °F

K = αCPρ


In either case, the typical properties of carbon and Generally, due to small particle size and high CTE, (1–Ø)K graphite generally yieldR' = theσƒ best overall thermal shock POCO graphite may not fare as well as some other resistance of all the temperature-resistant,αE non-metal- graphites in overall thermal shock resistance. Tests lic materials available. This is useful for rocket nozzle at Entegris have shown that samples of the DFP-1 and reentry nose-cone applications, among others. grade, measuring one inch cubed, can survive a 1000°C to 10°C instantaneous change without fractur- Sample Calculation: ing in repeated tests. Other tests on POCO graphites have subjected the samples to 335°F to -335°F instan- A graphite sample with a thermalKT conductivity of 0.29 (cal cm)/(cm2 sec R°C) = andS a tensile strength of taneous changes without affecting the graphite. A αE 10,000 psi (703 kg/cm2) has a Tmodulus of elasticity of comparison of some thermal shock resistance values 1.6 × 106 psi (0.112 × 106 kg/cm2) and a CTE of 8.4 × for some materials is shown in Table 12-1. 10-6 (in/in)/°C; calculate its thermal shock resistance. TABLE 12-1. THERMAL SHOCK RESISTANCES 0.29 cal × cm 703 kg cal/cm•sec KT cm2 × sec × °C cm2 R =S = -6 -6 POCO graphite (DFP-1) 217 αET 8.4 × 10 0.112 × 10 kg °C cm2 (SFG-1) 233 cal GLC (Graphnol) 450 R = 216.7 cm × sec Mersen graphite (2020) 140 Pyrolytic graphite (w/grain) 4300 Test Method (a/grain) 0.29 There are no standard test methods for accurately SGL Group, The Carbon (EK-82) 252 Company graphite measuring the thermal shock resistance of materials. (EK-87) 426 A number of practical tests are being used for specific applications. One such testing device is described by Titanium carbide (unknown) 3.45 Sato.9 Use of the relationships previously described Toyo Tanso USA graphite (Isograph 88) 340 are generally accepted for prediction of the thermal UCAR (ATJ) 329 shock resistance of a material, but many other consid- erations must be made such as sample size, stress While the differences are not as great between the distribution in material (i.e., geometry and stress various grades as the pyrolytic graphite shows between duration). orientations, it should still be noted that processing variables, starting materials, etc., can influence the POCO Graphite vs. Conventional Graphites thermal shock resistance of graphite. For example, the Some early studies10 reported that coarse-particle Ringsdorff EK-87 grade has an average particle size graphite had better thermal shock resistance than of 20 microns, yet it has about 30 percent more calcu- finer-particle graphite despite the higher strength of lated thermal shock resistance than the UCAR ATJ, finer-particle graphite. The study further indicated a which has an average particle size of 25 microns and correlation to the . If the binder were decreased, nearly twice the calculated resistance of a POCO the thermal shock resistance also dropped, suggesting graphite with a particle size average of 4 microns. the thermal stresses were being absorbed by the binder material. Other studies11 have shown the pre- Temperature Effects cursor material, e.g., tar coke versus oil coke, to have Depending on the type of graphite used and its partic- effects on the thermal shock resistance. The oil coke ular characteristics or properties, and considering had resistance values about seven times higher than shape, size, etc., higher temperature drops could be the tar coke. sustained without affecting the graphite. The tempera- ture starting and ending points may have some bearing 9 Sato, S., Sato, K., Imamura, Y. and Kon, J., Carbon, 13 (1975), on the overall resistance to fracture, because the p. 309. material will be thermally stressed differently at ele- 10 Kennedy, A.J., Graphite as a Structural Material in Conditions of High Thermal Flux. (Cranfield: The College of Aeronautics, vated temperatures. A drop from 1000°C to 500°C may November 1959. [CoA Report No. 121]), p.15. respond differently than from 500°C to 0°C. Little data 11 Sato, S., Hwaji, H., Kawamata, K. and Kon, J., “Resistance and is available to clearly define what the relationship or Fracture Toughness Against Thermal Shock of Several Varieties limits are for the different types of graphite. of Graphite," The 22nd Japan Congress on Materials Research – Metallic Materials, Mito, Japan: March 1979, 67-68.


Density Effects TABLE 13-1. TYPICAL SPECIFIC HEAT OF VARIOUS MATERIALS Since, generally, strength and thermal conductivity increase with density, higher thermal shock resistance Heat Capacity Heat Capacity should be found in higher-density ranges. However, Material Range cal/(g°C) Range J/(g K) with higher density, higher modulus and CTE are also Gold 0.031 0.13 usually found and these would tend to offset the gains Platinum 0.031 0.13 in strength. The coarser-particle systems may be more Tungsten 0.034 0.14 effective than the finer-particle systems on an equal density basis. Many factors are involved, and well Tungsten carbide 0.04 0.17 defined tests to measure these effects are not in Silver 0.056 0.23 common use. Brass 0.090 0.38 Nickel 0.091–0.14 0.38–0.59 Specific Heat Copper 0.092 0.38 Steel (wrought) 0.11 0.46 Definition Iron (cast) 0.13 0.54 Heat capacity is the quantity of heat required to raise Graphite 0.17 0.72 the temperature of a unit mass of material 1°, and the relationship between the two most frequently used Alumina 0.19 0.79 units is as follows: Aluminum 0.22–0.23 0.92–0.96 Silicon carbide 0.285–0.34 1.19–1.42 Btu cal 1 = 1 lb °F g °C For the temperature range 300 K to 3200 K:

-4 It has been found that the values of heat capacity for CP = 0.44391 + (0.30795 × 10 )T – (0.61257 × 105)T-2 + (0.10795 × 108)T-3 all types of natural and manufactured graphites are basically the same, except near absolute-zero tempera- This expression yields calculated values within 1.5 tures. The differences found in measuring the same percent of the experimental values for the entire grade of graphite are as great as the differences in temperature range. WS DS measuringCP (graphite) different =grades× of graphite. × CP (sapphire) Differences as much as nine percentW GhaveD beenG found between Table 13-2 represents the best fit of the values for typi- natural graphite and manufactured graphite at low cal manufactured graphite with proper consideration temperatures (120 K to 300 K), but at higher tempera- given to the accuracy of the individual measurements. tures the differences for all types of graphite has been found to be less than the experimental error. Table Sample Calculation: 13-1 shows comparisons to other materials. Determine the heat capacity of a sample of graphite at Btu cal 500°C using a differential1 scanning= 1 calorimeter (DSC) Heat Capacity Equation and a strip chart recorderlb °F to followg °C the calorimetric

The heat capacity at constant0.023 g pressure40 (CP) cancal be response for the temperature scans of 30°C. DSC scans CP (graphite) = × × 0.19 expressed by polynomial0.077 functions g 52.5 of the absoluteg °C are taken for the sample of graphite, a sapphire refer- temperature T, with T in K to give C in cal/(g°C). ence sample and the baseline for correction of the cal P J CP (graphite) = 0.382 = 1.6 measured response. The heat capacity, CP is then Within the temperature intervalg °C 0 K to 300g K:K calculated for the graphite using the formula:

-4 -5 2 WS DS CP = (0.19210 x 10 )T – (0.41200 x 10 )T – CP (graphite) = × × CP (sapphire) (0.10831 x 10-7)T3 – (0.10885 x 10-10)T4 WG DG

Where: W S = Weight of sapphire However, below 40 K, this equation has not been WG = Weight of graphite reliable in correlation with experimental data. The DS = Signal displacement of sapphire calculated values are in good agreement (<2.2 per- DG = Signal displacement of graphite cent) with the experimentalW valuesS,λ for temperatures En,λ = above 40 K. WBB,λ

0.023 g 40 cal CP (graphite) = × × 0.19 0.077 g 52.5 g °C 26 PROPERTIES AND CHARACTERISTICS OF GRAPHITE cal ENTEGRIS,J INC. CP (graphite) = 0.382 = 1.6 g °C g K

C 1 1 logEn,λ = – 2.303λ T TA,λ

WS,λ En,λ = WBB,λ WS En,λ = WBB

4 T A,t C 1 1 En,t = = T4 logEn,λ – 2.303λ T TA,λ

WS En,λ = WBB

4 T A,t En,t = T4 SPECIFIC HEAT Btu cal 1 = 1 lb °F g °C

TABLE 13-2. TYPICAL SPECIFIC HEAT OF Let the weight of the sapphire reference = 0.203 g and MANUFACTURED GRAPHITES a sample of DFP graphite, i.e., indicating a density of 3 WS DS Specific Heat Specific Heat 1.82 g/cmCP (graphite)and measuring = 1.5× mm × × C P6 (sapphire) mm diameter, WG DG Temperature (K) (cal/[g°C]) (J/[g K]) weigh 0.077 g, respectively. CP of sapphire is 0.19 cal/ 0 0.0000 0.0000 (g°C). CP of graphite at 500°C can be calculated by knowing the displacement as determined from the 50 0.0101 0.0423 strip chart recorder. 100 0.0335 0.1402 Assume the displacement (corrected) is 40.0 for the 150 0.0643 0.2690 graphite and 52.5 for the sapphire reference. 200 0.0995 0.4163 Therefore: 250 0.1357 0.5678 0.023 g 40 cal CP (graphite) = × × 0.19 300 0.1723 0.7214 0.077 g 52.5 g °C 350 0.2090 0.8750 cal J CP (graphite) = 0.382 = 1.6 400 0.2450 1.0258 g °C g K 450 0.2760 1.1556 500 0.3030 1.2686 Test Method 550 0.3230 1.3523 The previous example best illustrates how a DSC and 600 0.3400 1.4235 a chart recorder may be used in determining heat capacity. However, modern technology has integrated 650 0.3560 1.4905 the DSC with computer and software technology,

700 0.3700 1.5492 thereby eliminating the need for calculating CP from WS,λ a chart recorder. EntegrisEn,λ = currently uses a Netzsch 750 0.3820 1.5994 W STA-449 Heat Flux DSC (FigureBB,λ 13-1) for evaluating 800 0.3930 1.6454 graphite and other related materials. Heat capacity 850 0.4020 1.6831 calculations are made via software but the fundamen- 900 0.4090 1.7124 tal principles remain the same.

950 0.4150 1.7375 Gas outlet 1000 0.4210 1.7626

1100 0.4320 1.8087 C 1 1 FurnacelogE = – n,λ Protective 1200 0.4430 1.8547 2.303λ T TA,λ DSC sensor tube 1300 0.4520 1.8924

1400 0.4600 1.9259 Hoisting device 1500 0.4680 1.9594 Gas inlet 1600 0.4740 1.9845 1800 0.4860 2.0348 Balance WS En,λ = 2000 0.4960 2.0766 WBB 2200 0.5040 2.1101 2400 0.5110 2.1394 2600 0.5160 2.1604 2800 0.5210 2.1813 Figure 13-1. Netzsch STA-449 4 T A,t 3000 0.5270 2.2064 En,t = T4 3200 0.5360 2.2441 In DSC, a distinction must be made between the recorded baseline in the presence and absence of a 3400 0.5480 2.2944 sample. Thus, measurements are first taken with an 3600 0.5800 2.4283 empty sample crucible to determine the instrument 3800 0.6900 2.8889 baseline. A second run is made to verify the accuracy of the instrument by placing a sapphire reference sam- ple in the sample crucible and taking measurements


under the same experimental conditions. The DSC Density Effects curve rises linearly resulting from the change in heat capacity of the sample as a function of temperature. Heat capacity has no significant change as a function Sapphire is routinely used as a reference material of density. It is intrinsic to the graphite crystal itself since it is stabilized, i.e., will not oxidize, and due to and apparently independent of density. the fact that heat capacity for sapphire has been thor- 3.5 oughly investigated and well documented. Once the 3.0 baseline and accuracy have been established, a third run is made on the sample of interest under the same 2.5 experimental conditions. 2.0

There is no standard test method for determining 1.5 (J/g K) P C specific heat of graphite, but the DSC method is com- 1.0 monly used for specific heat determination of other Manufactured graphite 0.5 American Institute of Physics materials. Handbook, 3rd edition, 1972 Another simple method which yields approximate spe- 0.0 POCO DFP-1 graphite cific heat data is an ice calorimeter. If a body of mass I 01000 2000 3000 4000 (m) and temperature (t) melts a mass (m ) of ice, its Temperature (°C) temperature being reduced to 0°C, the specific heat of the substance is: Figure 13-3. Specific heat of manufactured graphite

2.5 Emissivity 2.0 TRA-1 (Taylor-Groot) Definition

1.5 A measure of the heat radiating ability of a surface is (J/g K) P

C referred to as emissivity. Total emissivity or spectral emissivity are ways to express it. The total emissivity 1.0 is defined as the ratio of thermal energy emitted by a material per unit time per unit area to that emitted by a black body over the entire band of wavelengths when 0.5 0 500 1000 1500 2000 2500 3000 both are at the same temperature. A black body, i.e., Temperature (°C) ideal radiator, is one which absorbs all the radiant energy falling on it and, at a given temperature, radi- Figure 13-2. Specific heat vs. temperature: POCO graphite ates the maximum amount of energy possible over the entire spectrum of wavelengths. POCO Graphite vs. Conventional Graphites The spectral emissivity is defined as the ratio of ther- mal energy emitted by a material per unit time per POCO graphites have heat capacity values that are unit area to that emitted by a black-body reference in the same range as conventional manufactured where radiation from both are of the same wavelength graphites. POCO graphites have a range from around and both are at the same temperature. The spectral 0.72 J/(g K) to 1.60 J/(g K) between 20° and 500°C emissivity of graphite has been measured at 6500 Å. (Figure 13-2).12 Dull surfaced graphite and polished graphite have had typical values of 0.90 and 0.77, respectively. It has Temperature Effects been determined that the emissivity of graphite does The heat capacity of manufactured graphite increases vary slightly with temperature. A temperature coeffi- with increasing temperature (Figure 13-3). cient of 1.9 × 10-5/K has been determined. Graphite emissivity has been found to be nearly constant in the wavelength range 2000 Å to 60,000 Å. Spectral emissiv- 12 Taylor, R.E. and Groot, H., Thermophysical Properties of POCO ity values approaching 0.99 have been measured for Graphite. (West Lafayette, Indiana: Purdue University, July 1978. graphite near sublimation temperatures of 3600 K. [NTIS No. ADA060419]), p.15.


Btu cal 1 = 1 lb °F g °C Btu cal = 1 1 WS DS CP (graphite) =lb °F ×g ×°C CP (sapphire) WG DG

WS DS CP (graphite) = Btu ×cal × CP (sapphire) 1 WG = 1DG lb °F g °C WS DS CP (graphite) = × × CP (sapphire) WG DG

0.023 g 40 cal CP (graphite) = × × 0.19 0.077 g 52.5 g °C

cal J EMISSIVITY CP (graphite) = 0.382 = 1.6 WS DS CP (graphite) = ×g °C × CP (sapphire)g K W D 0.023G g G 40 cal CP (graphite) = × × 0.19 0.077 g 52.5 g °C 0.023 g 40 cal CP (graphite) = ×cal × 0.19J CP (graphite)0.077 = 0.382 g 52.5= 1.6 g °C g °C g K For a specific temperature, at acal specific wavelength,J Levitt.13 A schematic of this equipment can be seen CP (graphite) = 0.382 = 1.6 normal spectral emissivity cang be °C expressedg K as: in Figure 14-1. The methods of measuring emissivity are wrought with difficulties and results may be biased WS,λ En, = λ W due to equipment and/or assumptions made in run- 0.023 g BB,λ 40 cal CP (graphite) = × × 0.19 ning the tests. At best, the testing is at a research Where: 0.077 g 52.5 g °C level and not a routine procedure.

Holders and electrodes WS,λ = Normal spectral radiant energycal emittedJ by a CP (graphite) = 0.382 = 1.6 specimen material per WunitS,gλ °C time, unitg areaK and Specimen En,λ = (C) unit solid angle. WBB,λ Total WS, Micro-optical E = λ radiation WBB,λ = Normal spectral n,radiantλ energy emitted by a pyrometer (O) WBB,λ detector black-body referenceC per unit1 time,1 unit area, and unitlogE solidn,λ = angle. – Vacuum/pressure 2.303λ T TA,λ chamber Normal spectral emissivity written as a function of temperature is: Millivolt Precision temperature potent- WS,λ recorder (M) iometer (P) En,λ C= 1 1 logEn,λ = WBB,λ – 2.303λ T TA,λ C 1 1 logE = – n,λ WS Where: C = 14,380 micronE2.303n,λ = –λ K T TA,λ WBB λ = 0.65 micron Power D.C. Vacuum Micro-optical T = Temperature of material (K) and controls pump and pyrometer (O) (E) gauges (V) TA,λ = Apparent temperature of material (K) For a specific temperature theW total normal emissivity Figure 14-1. Schematic block diagram of the emissivity apparatus = S can be expressed as: En,λC 1 1 logEn, = WBB – λ 2.303 T T λ4 A,λ POCO Graphites vs. Conventional Graphites TWA,tS EEn,λ = n,t 4 WT BB Very little information on the emissivity of graphite is available. The following information on total emit- Where: tance of two grades of POCO graphite and pyrolytic

WS = Normal total radiant energy emitted by a specimen graphite has been reported by Cezairliyan and 4 material per unit time, unitT A,t area and unit solid Righini.14 En,t = angle. TW4 = S En,λ 4 The equation for total emittance of POCO (TRA-1) TWA,tBB WBB = Normal total radiantEn,t =energy emitted by a black graphite as a function of temperature, between 1800 T4 body reference per unit time, unit area and unit to 2900 K is: solid angle. E = 0.679 + (6.00 × 10-5)T Total normal emissivity at elevated temperatures can be expressed as: The equation for POCO (DFP-2) graphite, between 4 1700 to 2900 K is: T A,t En,t = T4 E = 0.794 + (2.28 × 10-5)T

Where: T = Temperature of material (K) The equation for pyrolytic graphite, between 2300 to

TA,t = Apparent temperature of material (K) 3000 K is: Emissivity data has usefulness in applications such as E = 0.641 – (5.70 × 10-5)T aerospace, heaters, pyrometry, etc., but published data Where: T is in K. is difficult to find. 13 Grenis, A.F. and Levitt, A.P., The Spectral Emissivity and Total Test Method Normal Emissivity of Graphite at Elevated Temperatures. (Watertown, Mass.: Watertown Arsenal Laboratories, November There are no standard methods for measuring emissiv- 1959. [NTIS No. ADA951659]), p.14. ity of graphite, but one method used to measure the 14 Cezairliyan, A. and Righini, F., “Measurements of Heat Capacity, emissivity of graphite is described by Grenis and Electrical Resistivity, and Hemispherical Total Emittance of Two Grades of Graphite in the Range of 1500° to 3000°K by a Pulse Heating Technique,” Rev. in Htes Temp. et Refract., t.12 (1975), p. 128.


TABLE 14-1. TOTAL EMITTANCE Ash Temperature POCO POCO Pyrolytic (K) TRA-1 DFP-1 Graphite Definition 1700 — 0.833 — ASTM Standard C709 defines ash of manufactured car- 1800 0.787 0.835 — bon and graphite as the “residual product following 1900 0.793 0.837 — oxidation of the base carbon as determined by pre- 2000 0.799 0.840 — scribed methods.” This ash or residual product generally results from natural contamination of the 2100 0.805 0.842 — raw material used to produce graphite and may be 2200 0.811 0.844 — introduced to a lesser extent during processing of the 2300 0.817 0.846 0.510 raw material or graphite products. Table 15-1 shows 2400 0.823 0.849 0.504 a general listing of typical elements and their level of contamination in graphite bulk product. As will be 2500 0.829 0.851 0.499 discussed later, there are means by which the contami- 2600 0.835 0.853 0.493 nation can be reduced or removed. 2700 0.841 0.856 0.487 TABLE 15-1. TYPICAL PURITY ANALYSIS 2800 0.847 0.858 0.481 STANDARD POCO GRADES (UNPURIFIED) 2900 0.853 0.860 0.476 TOTAL ASH RANGE 300-1000 PPM 3000 — — 0.470 Element Element Detected ppm Range Vanadium (V) Yes 100–500 The measured values of the total emittance of the Iron (Fe ) Yes 5–300 graphite specimens are shown in Table 14-1 and again graphically in Figure 14-2. Nickel (Ni) Yes 1–100

0.87 Calcium (Ca) Yes 5–100 Silicon (Si) Yes 5–50

0.86 Aluminum (Al) Yes 10–50 Titanium (Ti) Yes 1–50 DFP-1 0.85 (K) Yes 1–20 (Na) Yes 1–15

0.84 Copper (Cu) Yes 1–10 Total Emittance Total TRA-1 Magnesium (Mg) Yes 1–5

0.83 Chromium (Cr) Yes Trace to 10 (P) Yes Trace to 10 0.82 Boron (B) Yes 1–5 1400 1600 1800 2000 2200 2400 2600 2800 Sulfur (S) Yes 1–5 Temperature (°C) (Mo) Yes Trace Figure 14-2. Total emittance of POCO graphite Zinc (Zn) Yes Trace The differences may be associated with layer plane Lithium (Li) Yes Trace orientation or other crystallographic effects, particu- larly when comparing a pyrolytic graphite to a Test Method polycrystalline graphite such as TRA-1. The difference seen between DFP-1 and TRA-1 may go back to a sur- The standard test method for ash content of graphite face roughness effect, but are primarily associated is ASTM Standard C561. This technique applies princi- with the porosity differences with regard to size and pally to nonpurified graphites of several hundred ppm, frequency distribution across the surface. or more, of total impurities. The method is generally too crude for good reproducibility on high-purity samples.


One of the disadvantages of this method is the poten- In material which has been impregnated for densifica- tial loss of low melting point impurities due to the tion, contaminants in the raw material used could temperature at which the test is run. Theoretically, at increase the impurity content as well. 950°C as a final test temperature, nothing should be POCO graphites will typically range from 300–3000 lost that was not already volatilized during graphitiza- ppm total impurities, depending on the grade and tion, but as a practical matter, it seems to occur. These density. Many conventional graphites fall within this are possibly impurities trapped in closed pores that range also, but some will be in the range of 0.1 percent did not successfully escape during the graphitization (1000 ppm) to several percent (20,000–30,000 ppm). cycle. The test temperature could be lowered to pre- clude some of this from occurring, but then the time Purified POCO graphite will have less than 5 ppm total to complete the test is extended considerably as oxida- impurities. This is one of the best grades of purified tion is a time-temperature dependent reaction. graphite available anywhere. The unique nature of the POCO pore structure and relatively clean raw materi- It has also been found in previous studies that at als aids in allowing this ultra-high purity. Typical higher temperatures (desirable for shorter test cycles) analysis of a purified POCO graphite is seen in Table reactions of carbon and platinum, of which the cruci- 15-2. Purified graphites can be produced in several bles are made, can occur and consequently bias the ways. Simply heat-treating to very high temperatures, test results. usually in excess of 3000°C, will volatilize most heavy Entegris’ method, TDI (Appendix B), differs elements present. Halogen gases, such as chlorine or slightly from the ASTM procedure, in that a solid sam- fluorine, will react with the impurities at high temper- ple is used rather than a powdered sample. The risk of ature and volatilize off as chloride or fluoride salts. contamination in powdering the sample is nonexistent Some elements such as boron are very stable and diffi- if a solid sample is used. A standard test cycle for cult to remove, especially since they can substitute for nonpurified material is 24 hours, whereas, a purified carbon atoms in the crystal structure. material usually takes 48–72 hours to complete the cycle. Temperature Effects The only real temperature effect on ash levels comes Sample Calculation: from the graphitization temperature or a subsequent If a graphite sample weighed 80.0000 grams after dry- thermal processing to remove impurities. As men- ing and was ashed in a 35.0000 grams crucible where tioned previously, the higher the final thermal the total ash after testing was 0.4 milligrams, the total treatment, the lower the ash level will usually be. ash would beAsh 5 ±1 (ppm) parts = Cper – A million. × 1,000,000 = Ash (ppm) = C – A × B1,000,000 – A = Density Effects B 35.0004– A – 35.000 × 1,000,000 = 5 ±1 ppm In POCO graphite, a slight trend has been noted 35.0004115.0000 – 35.000 – 35.000 with ash as related to density. Since POCO graphites × 1,000,000 = 5 ±1 ppm 115.0000 – 35.000 have increasingly more open porosity as the density decreases, more opportunity for volatilization escape Where: A = Crucible weight of the impurities during graphitization can occur. B = Crucible plus dried sample weight Consequently, slightly lower ash levels are typically C = Crucible plus ash weight found for the lower-density grades. This may be unique to POCO graphites. The difference is relatively small If ash % is required, multiply by C–A 100 though, so it is mentioned only in passing, as a number B–A Ash % = 0.0005 C–A 100 of other factors may have more significant effects on B–A the final ash level. POCO Graphites vs. Conventional Graphites

When discussing nonpurified Ingraphites, K α E/RT POCO graph- ites are fairly clean, i.e., contain relatively low levels of impurities. However, sinceIn K α graphitization E/RT temperature has a strong bearing on the impurities which remain, the ash levels on any product can vary accordingly. There are other process steps which influence the final ash level also.WTO–WTF 10.0000–9.9900 %WTL = × 100 = × 100 = 0.1% WTO–WTF WTO 10.0000–9.990010.0000 %WTL = × 100 = × 100 = 0.1% WTO 10.0000


2 Eo (1 – 1.9P + 0.9P ) 2 Eo (1 – 1.9P + 0.9P ) OXIDATION

Ash (ppm) = C – A × 1,000,000 = B – A 35.0004 – 35.000 × 1,000,000 = 5 ±1 ppm 115.0000 – 35.000 TABLE 15-2. TYPICAL PURITY ANALYSIS a gas (or gases) which dissipates and leaves behind PURIFIED POCO GRADES nothing except a small amount of ash, which is the TOTAL ASH RANGE 5 PPM OR LESS of the metallic impurities that were present Element Element Detected ppm Range in the carbon to begin with. Silicon (Si) Yes Trace The oxidation of carbon by oxygen (as well as other Sulfur (S) Yes Trace gases) is highly temperature-dependent.C–A 100 No detectable reaction occurs at temperatures up to about 350°C. Vanadium (V) Yes Trace B–A As the temperature is increased, the rate of reaction Calcium (Ca) Yes Trace increases rapidly, according to the well-known Boron (B) Yes Trace Arrhenius expression: Aluminum (Al) Yes Trace In K α E/RT Magnesium (Mg) Yes Trace Where: T = Temperature Iron (Fe) Yes Trace E = Activation energy Molybdenum (Mo) Yes Trace K = Reaction rate R = Universal gas constant Phosphorus (P) Yes Trace WT –WT 10.0000–9.9900 %WT = O F = = Nickel (Ni) No — TestL Method × 100 × 100 0.1% WTO 10.0000 Titanium (Ti) No — To measure the oxidation characteristics of a particular Potassium (K) No — carbon material, a piece of known weight is exposed to Sodium (Na) No — the “oxidizing environment” of interest for some period of time,Ash and (ppm) then = C reweighed, – A × 1,000,000 to determine = a weight Copper (Cu) No — loss (the oxidationB – weight A loss). There is not a widely Chromium (Cr) No — accepted standard35.0004 –test 35.000 for measuring the oxidation 2 = Eo (1 – 1.9P× +1,000,000 0.9P ) 5 ±1 ppm Zinc (Zn) No — characteristics115.0000 of carbon – 35.000 materials (TDI and in Appendix B). Lithium (Li) No — The oxidation characteristics of carbon are usually expressed in one of two different ways: Oxidation 1. The percent weight loss in 24 hours at a given tem- perature, often referred to as oxidation resistance Definition C–A 100 2. The temperature at whichB–A a sample loses approxi- Oxidation is a chemical reaction that results in elec- mately one percent of its weight in a 24-hour period, trons being removed from an atom, which makes the which is called the oxidation threshold temperature atom more “reactive” so that it may join with one or of the material being tested more other atoms to form a compound. When talking In K α E/RT about carbon and graphite, oxidation is normally Sample Calculation: thought of as the reaction of carbon atoms with If a sample originally weighed 10.0000 grams after oxygen to form carbon monoxide (CO) and carbon being dried and 9.9900 grams after oxidation testing, dioxide (CO2). However, many other gases (besides the percent weight loss would be 0.1 percent. oxygen) will react with carbon in an oxidation reac- tion (e.g., CO2, H2O, N2O, etc.). The result of the WTO–WTF 10.0000–9.9900 %WTL = × 100 = × 100 = 0.1% oxidation reaction is a loss of carbon atoms from WT 10.0000 the carbon or graphite material, which obviously O affects many of its properties and characteristics. If Where: WT = Initial sample weight (dried) oxidation of a piece of carbon is allowed to continue O WTF = Sample weight after oxidation testing percent long enough, all the carbon atoms will react to form WTL = Percent weight loss

2 Eo (1 – 1.9P + 0.9P )


POCO Graphites vs. Conventional Graphites increases. The “oxidation threshold temperature,” defined as that at which a sample loses approximately The oxidation characteristics of any graphite at the one percent of its weight in 24 hours, is about 570°C same temperature and atmospheric conditions are for purified graphite, while nonpurified graphite is dependent on the amount of impurities in the mate- about 430°C (Figure 16-1). rial, the density of the material and the amount of surface area available to react with the oxidizing The oxidation of graphite is highly temperature- atmosphere. POCO graphites can be impregnated with dependent. At temperatures up to about 350°C, no a proprietary oxidation inhibitor that can substantially detectable oxidation occurs. As the temperature is increase its oxidation resistance. Purification also increased, the rate of reaction increases rapidly, reduces oxidation significantly by removing metallic according to the Arrhenius equation (Figure 16-2). impurities, which act as oxidation catalysts. As the surface area increases, the oxidation rate Density Effects increases, too. This is expected, as the surface Since the oxidation of graphite is a surface reaction, exposure is important to the oxidation process. For it is known that the rate of oxidation is affected by purposes of standardization and to minimize the effect the porosity of the material. As the percent porosity of variable surface area to volume (SA/V) ratios, the increases, the apparent density of the material will use of a sample with an SA/V ratio of 10:1 is used at decrease. Therefore, as the apparent density decreases, Entegris for testing oxidation rates. the rate of oxidation will increase. However, the effects of impurities and their catalytic action can Temperature Effects easily override the density effects. Purified data shows the density difference more clearly, but it is, neverthe- The differences in oxidation behavior of the various less, relatively small when other factors are grades of graphite are widest at the lowest tempera- considered. tures, tending to disappear as the temperature

Oxidation Threshold Oxidation Rate Purified POCO Grades Purified POCO Grades 1000 1000

100 100

10 10

1 Threshold ight Loss/hour 1 ight Loss/24 hours % We

% We 0.1 0.1

0.01 0.01 300 400 500 600 700 800 900 1000 300 400 500 600 700 800 900 1000 Temperature (°C) Temperature (°C)

Oxidation Threshold Oxidation Rate Non-purified POCO Grades Non-purified POCO Grades 1000 1000

100 100

10 10

1 ight Loss/hour 1

ight Loss/24 hours Threshold % We

% We 0.1 0.1

0.01 0.01 300 400 500 600 700 800 900 1000 300 400 500 600 700 800 900 1000 Temperature (°C) Temperature (°C)

Figure 16-1. Oxidation threshold Figure 16-2. Oxidation rate


Appendix A LENGTH CONVERSION FACTORS From To Multiply By PREFIX CONVERSION FACTORS inch ft 0.0833 Prefix Symbol Unit Multiplier inch m 0.0254 giga G 109 inch cm 2.54 mega M 106 inch mm 25.4 kilo K 103 feet in 12.0 deci d 10-1 feet m 0.3048 centi c 10-2 feet cm 30.48 milli m 10-3 feet mm 304.8 micro µ 10-6 centimeter m 0.01 nano n 10-9 centimeter mm 10.0 pico p 10-12 centimeter µ 1.0 × 104 microns (µ) cm 1.0 × 10-4 DENSITY CONVERSION FACTORS microns (µ) in 3.937 × 10-5 From To Multiply By microns (µ) ft 3.281 × 10-6 g/cm3 lb/in3 0.03613 microns (µ) m 1.0 × 10-6 g/cm3 lb/ft3 62.428 microns (µ) Å 1.0 × 104 g/cm3 kg/m3 1000 Angstroms (Å) m 1.0 × 10-10 g/cm3 g/m3 1,000,000 lb/in3 g/cm3 27.6799 STRENGTH CONVERSION FACTORS lb/in3 kg/m3 27,679.905 Pressure: lb/ft3 g/cm3 0.016 From To Multiply by lb/ft3 kg/m3 16.0185 lb/in2 (= psi) lb/ft2 144.0 lb/in2 (= psi) g/cm2 70.307 lb/in2 (= psi) kg/cm2 0.0703 lb/in2 (= psi) kg/m2 703.07 lb/ft2 lb/in2 6.94 × 10-3 lb/ft2 g/cm2 0.4882 lb/ft2 kg/m2 4.8824 kg/m2 lb/ft2 0.2048 kg/m2 lb/in2 1.422 × 10-3 kg/m2 g/cm2 0.10 kg/cm2 lb/in2 14.223 g/cm2 kg/m2 10.0 g/cm2 lb/in2 0.01422 N/mm2 (= MPa) lb/in2 (= psi) 145.032 N/mm2 (= MPa) kg/m2 101.972 × 103 MN/m2 MPa 1 N/m2 lb/in2 (= psi) 1.45 × 10-4


ELECTRICAL RESISTIVITY CONVERSION FACTORS Appendix B From To Multiply by Ω-in Ω-cm 2.54 RESEARCH & DEVELOPMENT LABORATORY INSTRUCTIONS Ω-in Ω-m 0.0254 Number Instruction Ω-in Ω-ft 0.3333 Apparent Density of Carbon and Graphite Articles µΩ-in Ω-in 1.0 × 106 Electrical Resistivity of Carbon and Graphite Ω-ft Ω-in 12.0 Shore Scleroscope Hardness of Carbon and Ω-ft Ω-cm 30.48 Graphite Ω-ft Ω-m 0.3048 Rockwell Hardness of Graphite Ω-cm Ω-in 0.3937 Coefficient of Thermal Expansion (CTE) of Ω-cm µΩ-in 393.7 × 103 Graphite Ω-cm Ω-ft 0.0328 Ash Analysis Ω-cm Ω-m 0.01 Oxidation Resistance Test Method Ω-m Ω-in 39.37 Oxidation Threshold Test Method Ω-m Ω-ft 3.281 Flexural Strength of Carbon and Graphite Ω-m Ω-cm 100.0 Compressive Strength of Carbon and Graphite Ωmm2/m Ω-m 1.0 × 10-6 Polishing Samples for Photomicrographs Ωmm2/m Ω-cm 1.0 × 10-4 Microphotography and Examination of Carbon and Graphite COEFFICIENT OF THERMAL EXPANSION Permeability of Graphite Plates From To Multiply by Equotip Hardness of Carbon and Graphite (in/in)/°C (in/in)/°F 0.5556 1/°C 1/°F 0.5556 (in/in)/°F (in/in)/°C 1.80 1/°F 1/°C 1.80

Conversion constants for 1/°C to 1/°F and 1/°F to 1/°C are true for any dimensional unity, i.e., in/in, cm/cm, ft/ft, m/m.

THERMAL CONDUCTIVITY CONVERSION FACTORS From To Multiply by (Btu-ft/hr/ft2 °F) (cal cm)/(cm2 sec °C) 4.134 × 10-3 (Btu-ft/hr/ft2 °F) (Kcal cm)/(m2 hr °C) 148.8 (Btu-ft/hr/ft2 °F) (kilowatt hr in)/(ft hr °F) 3.518 × 10-3 (Btu-ft/hr/ft2 °F) (Btu-in/hr/ft2 °F) 12.00 (Btu-ft/hr/ft2 °F) (Btu-in/ft2/sec °F) 3.33 × 10-3 (Btu-ft/hr/ft2 °F) (watts cm)/(cm2 °C) 0.0173 (Btu-ft/hr/ft2 °F) (cal cm)/(cm2 hr °C) 14.88 (Btu-ft/hr/ft2 °F) (Btu-in/ft2 /day °F) 288.0 (Btu-ft/hr/ft2 °F) W/m-K 0.0173 1 calorie = 1 cal = 1 gram cal 1 calorie = 1 kilogram cal = 1 Kcal = 1000 cal


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